Carmax Wrecked, repainted, money pit. San Antonio Nationwide

Two years ago this month I purchased a 2009 mini cooper, 40k miles, from the Carmax in San Antonio, TX. I asked for a carfax report, salesman explained that any repairs done at a small shop probably wont show up so they are useless. My first mistake. Then they keep messing up the financing paperwork, we were there 6 hours, not walking away was my second mistake but i really needed a car. Third mistake was not keepjng the warranty after they gave me gave wrong one. About 13 months later the engine problems began. Turbo charger $3000, oil filter housing $1100, tire sensor $150,  blow off valve carbon build up $1367, rear light assembly circuit board caught fire $160. I have always taken my car to the dealership for repairs and maintenence, these are actually discounted prices from them working with me. I didn't list items that were covered by what was left of a manufacturer warranty. 

I am making $400 a month payments on a car that I am literally having to rebuild the engine part by part. Right now it has a small oil leak at the gasket which will cost several hundred dollars to fix and did I mention that my car now has only 56k miles. Unfortunately, my mom cosigned and I can't just dump it or I would. Anyone have some sage advice?