What do you do if you can't afford your car payments anymore?

Sell it on your own. You may not be able to sell it for the amount owed, but work out a deal with the bank to finish paying for the vehicle at a reduced rate after it's sold. Most lending institutions would much rather work out something than have it go into bankruptcy. That, plus it's a lot better for YOUR credit history if you take matters into your own hands and deal with it now.