> At a 5.5% interest rate - why is 15% of my monthly payment going to interest? Why shouldn't it be... 5.5%?

Because that's not how that works. 5.5% means that you pay 5.5% of your current balance in interest. A basic example:

You have a $12,000 car payment. Every month, you owe interest. 5.5% of $12,000 is $660. This is an annualized number, so dividing it by 12 gets us the monthly number. Which is $55.

Now, your monthly payment might be $100, of which $55 is interest. It might be $200, of which $55 is interest. That doesn't matter. The 5.5% is 5.5% of the balance; it's not a proportion of the payment.