A little over two years ago my girlfriend cheated with her ex.  He is also part of a larger friend group so we have run into him at social outings a handful of time since the incident.  I confronted him very early after the incident and had an adult conversation, though I think he’s a piece of shit still.  My girlfriend initially lied to me about it–giving my a shade of the truth until I found out the entire truth and she finally came clean.  It has been 3 years and I have been killing myself to stay in the best shape of my life out of some sort of anxiety that she is not longer attracted to me (thinking that the reason for cheating was a lacking in me).  Needless to say I am struggling with self confidence.  I hate myself periodically, especially when I feel unappreciated by my girlfriend.  I get angry when we encounter him in public and I am furious with her friend group for not kicking him out after what happened.  Everyone is content to sweep it under the rug because it did not affect them at all.  I am miserable and not sure how to confront the issue of seeing him (reopening the wound periodically) and not being able to deck him, and I am wearing my body down with occasional injuries to maintain what I think is a desirable physique for her.  I know it is 99% in my head but I was truly devastated and I don’t think I was hard enough on her during the 6-month period after her infidelity.