Having the vehicle registered in her name is ***** ***** ownership.
You can argue that she did not pay for it and that it is yours and show that you paid for it. The problem you have though is, declaring that the transfer was not legitimate and that she held is for you is fraud and you can be sued for that and be asked to pay to the departments the benefit you were receiving.
This is the biggest issue for you.
Alternatively you can sue her for the value of the car because she has not paid you and get an order form the court to seize the car once you have judgement.
The following link is a guide on how to sue her for the money she owes you for the car.
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