Lying boyfriend

Posting multiple places for advice.

I just need some advice to calm me down or on what to do. Recently got back with my ex after a few months due to him pretending like he changed and things would be different. He sure proved me wrong. He has this female “BFF” that he swears is just a friend but to me it seems as though it is more than that. She constantly tries to trigger me with posts on social media, never refers to me by name when talking to him (“that girl”) Nd calls whenever she so feels like it. She is borderline obsessed with him and I really believe they have something going on. When we got back together he asked what I needed from him to make things work and stated he would do anything I asked of him for it to work. I simply said that friendship has to end until she is able to respect my place in your life and our relationship. He agreed and promised me things would be different. Saturday was our two year anniversary and he told had to go out of town for his kid and told me he was not able to come visit. On this same day the best friend decided to post a picture of the two of them with a caption like “I’m trying to do life” completely crushed me. I sent to him and he laughed it off saying it was nothing and it was an old pic. And sent his location to prove he was out of town and not with her. He also stated that they are best friends til they die etc. He kept pleading the case and begging me to relax and telling me how he doesn’t wanna lose me. But I’m not buying it so I broke up with him. But this is still not sitting right with me and I feel like a punk for not saying something to the girl but I also am like is it even worth it. She can have him but I’m pissed by the disrespect. And I’ve never done anything to this girl. Any advice would be cool.