Well, if she wasn't your ex at that point at least you got the chance to dump her. If I knew about this, and I had the opportunity, I'd take her to an expensive, packed, restaurant (driving both of us), order the food, get on one knee and then say 'you've been a b*tch to me this whole relationship, would you do me the honor of being my ex'. Then I'd just leave her with the bill, pack my stuff, and leave.

If their name is on the lease too they can’t just ditch you. Get their name on the lease so you can legally go after them. If you have to you can even have it outlined in the lease how much each of you would pay.