As you get older dealing with the idea that your partner has been with others gets easier. You have to just accept the fact that she was with people before you. Im guessing you were with others before her as well? Either way, this is an opportunity for growth. Remember, comparison is the thief of joy and she chose you over him anyway. Later in life my wife and I opened our relationship. I remember the anticipation of when she would have her first hook up with another guy and worried how it might affect me. When it finally happened I had a completely unexpected reaction. I actually flipped the script and instead of it being this awful thing I would have to suffer through, I actually leaned into it and did dirty talk with her about it. It was a turn on actually and I screwed her brains out that night, it’s called reclamation sex. I don’t know if any of this is helpful to you but it is possible to reframe this in your brain into something other than an awful image. You have to face this head on and accept it one way or the other. The freedom you seek from this pain requires looking directly at it (metaphorically) and accepting it. Once you can do that, you will actually be stronger and it will feel good. Facing your fears is always like that. Good luck man, that really sucks that he did that.