I felt the same way at your age. I reduced my credit lines and canceled unused cards. It will hurt you to cancel this card 10 years from now. Discover is one of my oldest cards and thankful I didn't cancel it years ago. You can keep the card stuck away or just cut it up if you have to. Just don't cancel the account. Yes it takes some responsibility because you will still have to check discovers site each or other months to make sure no fraud charges are there. I do that for all my cards monthly. You can probably set up e-alerts for charges so you don't have to check it all the time. That $20k CL is your friend. It shows that you can control spending and not use it. Not using it is also part of your score. Your goal is to have 10-25x what ever you normally spend for a CL. You are protected from fraud up to 90 days so check your card at least ever 2 months if no activity. I pull my credit report at least monthly. Thats even enough to keep an eye on your accounts. Now if you can't fight temptation of spending then you have to reduce the CL. Maybe you can't sleep at night. I found most fraud hits up for a few hundred and then they stop using the cards. $2,000 or $100,000 CL on a card will still probably only get hit up to $2,000. But what ever you do, keep this card for life if you can even if you reduce the CL to $500. Never cancel your oldest card unless it costs you money. You will remember reading this post in 10 years when you still have this card holding up your scores for mortgages.. 
I'm a n00b here and I thank all of these great people who post because they are a wealth of information. I just recently went through a little credit hiccup when my ficos dropped below what I'm used to seeing. Here's some recent revelations I made while working on my credit scores, and thankfully now I'm back to all above 720's. Keep in mind that doesn't make me an expert like the great people here just makes me aware.
Your question about open credit lines. Lenders and your fico score like to see that you use credit but that you use it responsibly. Having high credit limits is a good thing especially if you ever have to use your credit cards for an emergency and you don't want your credit to suffer for it. Here's an example:
Based on the fact that creditors like to see your credit used to credit limit utilization at or about the national average of 7% on any given trade line you can then use that formula on your current revolving cards.
Lets say you have a 20,000 dollar credit line. You can safely charge on that card without impact to your credit score 1,400 dollars. That's not a lot when you stop and think about it. You could easily use that up if lets say your car breaks down or you need a new bed.
In answer to your question I wouldn't lower the credit limit on your card(s). You've earned them and that's why they are there and keep in mind the creditors like to see them and they impact your fico scores positively if not abused.
In a perfect world you always want the lowest possible credit utilization but we don't live in a perfect world and the creditors and CRAs don't play by the perfect rules. Here are some more helpful hints you may or may not know.
Open trade-lines. If you have more than 5 you will be scored down for that. Pay off trade-lines or balance your debt onto less than 5 trades then pay it down so it's less than 30% of your combined credit lines. It hurts your fico scores less to have a trade line at 50% usage than it does to have more than 5 trade lines with balances. Fico sees a lot of trade lines open as mis-use of your credit. Consolidation I guess is the key here and it helps your credit score. Plus all the open lines with zero balances help your credit score so it's a double win.
I've read here about a lot of folks using their cards for monthly bills and then paying them off each month. This is a risky business if you don't want it to impact your score and it differs with each person and creditor creating a lot of variables. The one sure thing is that if your monthly bills are less than 7% of your available credit then it won't hurt at all but will help because fico likes to see recent credit usage and scores it positively. (read: responsible credit usage) I'll give you some food for thought here and then let you make up your own mind about this.
Most credit card creditors play fair and by this I mean they report to the CRAs at the time your monthly statement is generated. This does two things. It allows you to charge for the month and pay it off by the due date without it appearing on your credit report. Remember that your due date is usually always before (read: day or days) your statement date. In this scenario where you've paid on or preferably before the due date your statement will cut and show a zero balance and that zero balance is reported to the CRAs. No harm no foul situation but it doesn't do anything for fico because remember fico likes to see responsible credit usage. If you want positive points for usage you'll need your creditors to report a balance ideally less than 7% of your credit limit and by all means less than 30% of your combined credit limits. For more positive points from fico pay it off the next month at your due date and your creditor will go back to reporting a $0 and that's healthy.
Now lets mention the creditors that play by their own rules. I won't mention names but there are trade lines that report without regard to the statement date. These creditors can either help or hurt your score. In the scenario where you pay your bills every month with a card like this and your total monthly bills are 90% of your credit line but you pay them off every month it going to affect your credit score if your creditor reports mid cycle between your statements. In this scenario you're going to get negative points from fico because the useage to credit line is too high. So know your cards and know which ones play by the rules and which ones don't. I had to learn this the hard way because there's no magic rule and the customer service people at your creditors won't tell you unless you talk directly with their credit department (read: the people who manage the reporting to the CRAs and such). The only place I found these dates was on reports pulled from TrueCredit.* (I'm not selling their service just saying that's where I found them maybe others know of some other place.) Even there you have to be crafty to find the date of report to the CRA. If you get a triple report there you can see what each of them are showing as the report date. Why I say you have to be crafty is because some CRAs like Equifax only report month / year, but others will report month / day / year.
Anyway the bottom line here is know your creditors as well as they know you. Use your cards to your benefit not theirs.
One final thing and then I'll put this response to bed. There's more information on a credit report than just what you see on most "consumer" stripped reports. As an example Experian keeps a record of your credit card usage as reported by the creditors each month. So lets say you have a card that's 5 years old a creditor can see what the balance was each month it reported. They can see periods of high usage, no usage, and responsible usage. The thing that kills me about this is that if you make a big purchase they can see how long it takes you either to pay the balance or slowly eat away at it. Remember responsible usage is key and the CRAs report it, don't think for one minute that any creditor doesn't know everything about your credit usage even if you go in with zero balances on everything thinking how will they know I was 40K in debt 2 months ago. Another complaint I have is that some CRAs allow for a "worst trade" mark to be placed on your file. So lets say you've had some problems in the past, you've paid them off and had them removed or you allowed them to run out the statute of limitations (SOL) on your file. If a creditor has flagged you prospective creditors can see it. You can have great credit and 5 worst trades and you'll find yourself wondering why you got denied. I found this little industry secret by reviewing my personal report one of my lenders gave me which is highly frowned upon in the credit industry because they don't want you to know about all their secrets or we'll figure out a way to fix those too and make our files completely accurate. Luckily I didn't have any but when I saw the line that said "worst trade = 0" I immediately had to ask what it was. One thing I can say about this is again responsible credit. If you get into trouble work with your creditors to get out. If you let something go to collections and charge off status you may get flagged worst trade and those are impossible to fix because there's no way of telling who placed it and if you did know and called them they'd probably deny that they have that ability anyway, but they do.
Always check your credit reports. Remember the CRAs are a repository of the information given to them by your creditors. If your creditors report to the CRAs a wrong address, spelling of your name, etc those need to be fixed. Fix them with the CRAs and then check with all your creditors to make sure they have the right information on file for you. Creditors like to see all the personal information match up. They also like to see a phone number in your personal information as it helps provide them with stability and as we all know stability and responsibility are the two key factors in any FICO score.
Okay that's it from the peanut gallery I hope I've helped share what I've learned.
- AZQ
PS - Want an immediate jump in your score? Pay 10% of your outstanding revolving debt in any one month and watch your numbers climb.
Your question about open credit lines. Lenders and your fico score like to see that you use credit but that you use it responsibly. Having high credit limits is a good thing especially if you ever have to use your credit cards for an emergency and you don't want your credit to suffer for it. Here's an example:
Based on the fact that creditors like to see your credit used to credit limit utilization at or about the national average of 7% on any given trade line you can then use that formula on your current revolving cards.
Lets say you have a 20,000 dollar credit line. You can safely charge on that card without impact to your credit score 1,400 dollars. That's not a lot when you stop and think about it. You could easily use that up if lets say your car breaks down or you need a new bed.
In answer to your question I wouldn't lower the credit limit on your card(s). You've earned them and that's why they are there and keep in mind the creditors like to see them and they impact your fico scores positively if not abused.
In a perfect world you always want the lowest possible credit utilization but we don't live in a perfect world and the creditors and CRAs don't play by the perfect rules. Here are some more helpful hints you may or may not know.
Open trade-lines. If you have more than 5 you will be scored down for that. Pay off trade-lines or balance your debt onto less than 5 trades then pay it down so it's less than 30% of your combined credit lines. It hurts your fico scores less to have a trade line at 50% usage than it does to have more than 5 trade lines with balances. Fico sees a lot of trade lines open as mis-use of your credit. Consolidation I guess is the key here and it helps your credit score. Plus all the open lines with zero balances help your credit score so it's a double win.
I've read here about a lot of folks using their cards for monthly bills and then paying them off each month. This is a risky business if you don't want it to impact your score and it differs with each person and creditor creating a lot of variables. The one sure thing is that if your monthly bills are less than 7% of your available credit then it won't hurt at all but will help because fico likes to see recent credit usage and scores it positively. (read: responsible credit usage) I'll give you some food for thought here and then let you make up your own mind about this.
Most credit card creditors play fair and by this I mean they report to the CRAs at the time your monthly statement is generated. This does two things. It allows you to charge for the month and pay it off by the due date without it appearing on your credit report. Remember that your due date is usually always before (read: day or days) your statement date. In this scenario where you've paid on or preferably before the due date your statement will cut and show a zero balance and that zero balance is reported to the CRAs. No harm no foul situation but it doesn't do anything for fico because remember fico likes to see responsible credit usage. If you want positive points for usage you'll need your creditors to report a balance ideally less than 7% of your credit limit and by all means less than 30% of your combined credit limits. For more positive points from fico pay it off the next month at your due date and your creditor will go back to reporting a $0 and that's healthy.
Now lets mention the creditors that play by their own rules. I won't mention names but there are trade lines that report without regard to the statement date. These creditors can either help or hurt your score. In the scenario where you pay your bills every month with a card like this and your total monthly bills are 90% of your credit line but you pay them off every month it going to affect your credit score if your creditor reports mid cycle between your statements. In this scenario you're going to get negative points from fico because the useage to credit line is too high. So know your cards and know which ones play by the rules and which ones don't. I had to learn this the hard way because there's no magic rule and the customer service people at your creditors won't tell you unless you talk directly with their credit department (read: the people who manage the reporting to the CRAs and such). The only place I found these dates was on reports pulled from TrueCredit.* (I'm not selling their service just saying that's where I found them maybe others know of some other place.) Even there you have to be crafty to find the date of report to the CRA. If you get a triple report there you can see what each of them are showing as the report date. Why I say you have to be crafty is because some CRAs like Equifax only report month / year, but others will report month / day / year.
Anyway the bottom line here is know your creditors as well as they know you. Use your cards to your benefit not theirs.One final thing and then I'll put this response to bed. There's more information on a credit report than just what you see on most "consumer" stripped reports. As an example Experian keeps a record of your credit card usage as reported by the creditors each month. So lets say you have a card that's 5 years old a creditor can see what the balance was each month it reported. They can see periods of high usage, no usage, and responsible usage. The thing that kills me about this is that if you make a big purchase they can see how long it takes you either to pay the balance or slowly eat away at it. Remember responsible usage is key and the CRAs report it, don't think for one minute that any creditor doesn't know everything about your credit usage even if you go in with zero balances on everything thinking how will they know I was 40K in debt 2 months ago. Another complaint I have is that some CRAs allow for a "worst trade" mark to be placed on your file. So lets say you've had some problems in the past, you've paid them off and had them removed or you allowed them to run out the statute of limitations (SOL) on your file. If a creditor has flagged you prospective creditors can see it. You can have great credit and 5 worst trades and you'll find yourself wondering why you got denied. I found this little industry secret by reviewing my personal report one of my lenders gave me which is highly frowned upon in the credit industry because they don't want you to know about all their secrets or we'll figure out a way to fix those too and make our files completely accurate. Luckily I didn't have any but when I saw the line that said "worst trade = 0" I immediately had to ask what it was. One thing I can say about this is again responsible credit. If you get into trouble work with your creditors to get out. If you let something go to collections and charge off status you may get flagged worst trade and those are impossible to fix because there's no way of telling who placed it and if you did know and called them they'd probably deny that they have that ability anyway, but they do.
Always check your credit reports. Remember the CRAs are a repository of the information given to them by your creditors. If your creditors report to the CRAs a wrong address, spelling of your name, etc those need to be fixed. Fix them with the CRAs and then check with all your creditors to make sure they have the right information on file for you. Creditors like to see all the personal information match up. They also like to see a phone number in your personal information as it helps provide them with stability and as we all know stability and responsibility are the two key factors in any FICO score.
Okay that's it from the peanut gallery I hope I've helped share what I've learned.
- AZQ
PS - Want an immediate jump in your score? Pay 10% of your outstanding revolving debt in any one month and watch your numbers climb.