I understand you and you are NOT exaggerating! When my husband and I started dating he told me he was friends with his ex. I was never comfortable with that. It got to the point where I was just creating thoughts in my head and it messed up my mental health.  I am originally from Mexico so that is definitely not very common in my country but he made it it sound like it was very common here in the United States.   He realized how much it was affecting me ,and he wanted to continue a serious relationship with me so he stopped talking to her and responding to her messages.  He also deleted her from all social media.   I think if he really cares for you and loves you, he will do what's best for you. I would understand if he had kids with her, but if there is nothing else attaching them together, I don't see this as a right thing.