I credit score dropped a lot after I forgot to pay for my Discovercard which only have balance less than $100. Hate them since they don't have auto payment option. I cancelled that card afterwards.
Is there anything I can do to improve the score? I don't know how such a small mistake can impact my score that much.
Any advice is sincerely appreciated.
You're biting off your nose to spite your face. Closing this card may have damaged your scores even further. You were the one who was late, it's not Discover's fault.
Most banks have online access and are able to accommodate auto payments.
Had you kept that card open, they may have been in the mood to offer you a goodwill adjustment after a certain amount of on-time payments.
I'd call them, beg forgiveness and ask them to reopen your account.