Even if a court wasn't involved you couldn't just take the car from her even though it is in your name. There is a contract at some level and taking the car as you suggest would surely be a breach of that contract.
Whether she would be due any money if you took the car is not determinable from
the minimal information you have provided. If there is equity she would surely be owed that. She could be due other damages caused by taking the car as well such as a short term rental until a repalcement is obtained.
So, in short; no, you can't just take the car because you changed your mind.

