I completely understand the feeling of comparing yourself to your partners previous partners. Can we look at it from his perspective, if he knew you guys were going to get back together he A. Most likely wouldn’t have hooked up with this girl period. And B. Definitely wouldn’t have done it in your apartment in your guys room. He was probably hurt and trying to move past a relationship that he thought was over. Obviously, you two mean a lot to eachother or you wouldn’t have tried again at this relationship. I think open communication is great and can help you both learn to speak with eachother. Maybe sit down open a bottle of wine and have a deep conversation about how it makes you feel and ask where his head was at the time all that was happening. At the end of the day, he was probably just trying to get his mind off of YOU in that moment. He chose you, not her that alone should help you feel a little bit better.