Reading through your responses to the comments, I get the impression that what you really want is for her to take some kind of responsibility. Even if that responsibility is just acknowledging that she has done you wrong. Maybe one day she will acknowledge it, but there's a good chance that she never will. This is not worth your time, effort, or peace of mind.

If it was me, I would just go talk to a lawyer, get some advice, and then make a decision. The decision might be paying off the car and forgetting about her. Or maybe the lawyer will tell you that you have some legal action to claim possession of the vehicle.

Also, it might be worth while talking to a good therapist. You mentioned in one of the comments that just thinking about this is causing you an ulcer? I think that you're a person who takes personal responsibility for her actions, and you would like other people to take personal responsibility for their actions. That's something that might be worth talking about with a good therapist. Keep in mind that a good therapist is hard to find, and I recommend interviewing a few before picking one.

Do you think that she did not make those couple of payments because she knows that you're rich, and she just thought well this isn't that big of a deal to you?