How do I get a car that i bought for my ex when I'm the only one on the loan and she has the insurance on her name

First, write a demand letter asking for the return of your vehicle.  If you don't get it back, then sue for the return of your vehicle.  If it is worth less than $10,000, then you can sue in small claims court.  If it is worth more than $10,000, then you will have to file in regular Superior Court.