A very important question is, "When or is your loan maturing (meaning, is it requiring you to pay more than the minimum payment)?" Do you have the option to make 3 different payments, such as, a) Minimum payment, b) Interest only or c) Full 30 year amortized payment? If so, then you can pay any of those amounts. Usually the bank has set it up where any payment above the minimum first pays interest, then anything above the minimum+interest would obviously pay down the principle. Because the idea is that if you don't pay the interest, you are adding to the principle. So it goes to say that if you paid principle (and not interest) you wouldn't be making a difference on the total principle owed, because the interest just gets tacked on.
If you are interested in additional advice, with no obligation, I would recommend calling a lender. I have worked with Kevin Piel at Terra Mortgage. His number is 714-342-0002.
Best of luck,
Robert M. van der Goes