Walk away. I know it's hard because you love him, but he doesn't love you. If he's with someone but wants to cheat on her with you, then he would do it to you if you dated.
I train horses everyday and what the problem is ground manners. What you should do is out her halter and lead rope on.
Walk her around, and then just stop if she takes a step or goes to make her back up 10 feet.
It's something you have to do frequently.