How do I end a contract with an auto lender who is charging excessive interest?

Purchased a vehicle in 9/2007 using a lender that the dealership termed "subprime." The sticker price was $14,999. I have since made 53 payments at $342.05 per month totalling $18,128.65. After 5 years of payments the principal has only reduced by $5,000 which shows that the lender has only applied $1,000 per year to my remaining principal balance. At this rate, I will end up paying $18,128.65 x 3 by the time the vehicle is paid off. In other words, almost $60,000 for a vehicle that only cost $15,000. Isn't this extortion/usury? Please advise.