He doesn't sound like someone that is capable or willing to be in a genuine, honest relationship. At the rate he is going, he might not get married because his fiancée may break it off because of his behavior. Regardless of what happens, I don't think you should compare yourself to her. She's not better than you because he's with her.

I don't think he has anything meaningful to offer you except pain, regardless of whether he marries or not. If you were his fiancée, he would be lying to you and whatever else he's up to. I think you're in a better position than her.

Love yourself more and cut ties with him. There are so many people out there and you can look back on this one day and say you dodged a bullet. He doesn't deserve you.