Nothing is wrong with you. I'm guessing she puts up with a lot more crap than you did? He had no problems cheating on you with her, do you really believe he's staying faithful now?
My ex is with one of the girls he cheated on me with. Their relationship looks good from the outside, but I've been in her position, I know it sucks. She just thinks that's the best she'll ever get and let's him get away with everything.
I understand the pity party though and I'm sorry for that. He sucks. She sucks. You're awesome