Good advice above^^^.
You can go to the bankrate.com auto calculator to download the entire amortization schedule that remains so you are working with something accurate.
Also, to the OP, it is admirable that you recognized that a refi wasn't going to work for you. Once you get through this loan entirely you will have an asset for the next vehicle loan. Even if it is not much of an asset valuation wise, at least you won't be carrying over neg equity to the next loan and will be able to get the next vehicle under much better terms.
One other relatively painless trick:
Pay the loan 1 week early.
Try to keep doing this each month. eg. Month one 1 week before due date, so three weeks after last payment. Month 2, 2 weeks "early" but 3 weeks after last payment. If you can do this you will be shocked how much interest you will save. On a simple interest loan, you only pay interest on the principal outstanding and your future principal/interest payment ratio changes for the subsequent month. If you need to take a month "off" just pay 4 weeks from the last payment and start over. You will still get the benefit of the reduced total principal amount for the remaining life of the loan.