Kira
Then to be blunt? Dump him. It'll hurt, sure. Then when she messes him around just tell him that you told him so and leave him to sort out his own problems. Then after all that, IF he grovels enough and IF you haven't moved on with someone else, then maybe take him back. Be the one to end it, not him. And make it clear why you're doing it. It's the only way you can walk out of this with your head held high. Don't get upset, get angry.