My fiancé is still close friends with the girl he left me for

My fiancé and I have have been together 6yrs engaged for 3. We have 3 kids together. When our youngest was 3 months old he walked out on us to be with someone from work. He cut all ties with me and our babies. I know the girl. We have shared immediate family because of this after they broke up we was forced to interact at a few family functions. We have a lot in common. A lot! We actually became good friends. Couple months later He came back home wanting us to make things work. They are still friends. I understand they work together but they work in two different buildings yet he goes to see her. I just found out that a month ago when he told me was at a guy friends house at 3am he was actually with her and her friends. He lied because he knew I’d be mad. Am I wrong for being pissed at them both?! Am I wrong for feeling like we are in some twisted sister wife situation? I’m not okay with any of this. Is it wrong of me to tell him not to talk to her anymore outside of work? This whole thing has me completely insecure and question our relationship and his feelings for me daily. Sigh. I know I’m stupid for staying and entertaining this foolishness but we have three babies… how do I just forget that they had a thing and move on?

Personally I feel if you guys have been engaged for 3yrs and he has already walked out on you, still seeing her and lying to you about it…well he has no intention on marring you. I think you know you need to move on for yours and your children sake but you just don’t want to admit it. Staying for the sake of the kids is never a good idea if he isn’t willing to change. Your kids are just going to grow up thinking this type of relationship is healthy when you know its not.