Most Helpful Guy
Your ex is a pig plain and simple and he believed with your long history together that you be weak enough to continue sleeping with him despite him having a new piece of ass he clearly doesn't care about. You proved him to be right.
a lot of people have special feelings for their father of their children or exes in general that bring back emotions and feelings. You have to harness that and not be sex toy for him he can go to when not with his fiance.