if you have stock parts for it, return the car back to stock complete, part out everything you can off of it that is aftermarket, and see where you stand then, taking a $500 loss now would be better than not making your payment and screwing your credit up. Worse comes to worse, borrow money from a family member, that will allow you to pay them back, even offer some interest on the money you borrow from them, PAY THEM BACK the moment you can, and drive something like th $5k focus, it will be reliable, and anything that might break is an easy fix.
I will second the notion that moving in with a your GF when you are young is retarded.