To help you visualize where your payments are going, at $15K maxed out, assuming a 2% minimum payment, you're paying $300/mo and ~$200 of that is interest. Only $100 goes to the principal of the loan.
At $14k balance, it's a $280/mo minimum payment, of which ~$186 is interest ($94 goest to principal).
At $10k balance, it's a $200/mo minimum payment, of which ~$133 is interest ($67 goes to principal).
Do you see, that by paying only the minimum payment, the amount being applied to principal is only half of that applied to interest, and as the loan balance goes down, the absolute amount of a minimim payment that is applied to principal also decreases (100 -> 94 -> 67).
One strategy for paying down principal is to commit to paying down a fixed amount of principal every month, like $200. An easy way to calculate such a payment is to look at the transaction history of your loan after each statement is cut, and see what the interest charge was on the last statement. Add $200 (or whatever value you decided) - that's how much you pay this month.
Paying $200/mo principal on a $15k loan should take around 6 years. Paying $300/mo principal cuts that down to about 4 years.
