Can they take my phone back if i paid for it?

I payed 200 so far on a iphone 7 it was under my ex girlfriend name, i promise to pay he back for it but she treated to take it back and she threated me about it. I don't know what to do since i paid some of it, and i don't have money to pay for another phone

If it is in her name it belongs to her

You paid the monthly charge while you used the phone. You didn't buy an interest in the phone itself. You must return it to her and find a way to obtain a different one.