what do you think i should do?

i let my friend drive my car for like 7 months because hers broke down. i told her she could buy it from me for 500 plus the tabs and then another 100 for my brand new stereo. while she was driving we paid the insurance and stuff. we agreed to $50 a paycheck, (every 2 weeks) well she gave me one payment, then 2 weeks ago, she said she didnt have the money, which would have been ok except for the fact she waited until the day i was supposed to get the money to tell me she didnt have it. she knew i needed the money because our paycheck was messed up and i needed to buy groceries and stuff. we have done lots of other stuff for her when she needs it. we got her a washer and dryer and some furniture and when i had it i gave her some money so she could get insulin. i never asked for anything in return, just that she pay me the $50 every 2 weeks for the car. other friends of mine say i should take my car back because she is being difficult about giving me my money and that she is taking