He slept with his ex on our break...

I just found out my boyfriend slept with his ex on our break.... Me and him hadmve been together 3 years. He was the guy I lost my virginity to. We had been fighting a lot. I was stressed because I have to work. He doesn't have to worry about working because his parents pay for everything as long as he stays in school. So while I'm stressed working my ass off he gets to hang around the lounge on campus. During one of our fights I said we needed to go on a break. He didn't want to but after talking he agreed to a break. We were on a break for almost 6 months. In those 6 months I wanted to have an actual sexual experience since I never had sex with anyone but him. In those six months I slept with 13 people. I'm not ashamed of it. I was using both birth control and condoms and when me and my boyfriend got back together we both go tested. I had asked him if he slept with anyone on our break and he said yes. He slept with one person on like month 4 of our break. When I asked whp and he said her name I got pissed. Nasty fucking Cassidy. His bitch ass ex. I fucking hate that bitch. Before anyone says "Well you slept with someone too". I wouldn't care if he fucked 100 people. Its the fact that he fucked an ex! I wasn't going finding any of my old boyfriend's and fucking them? I'm just mad and not sure what to do about it.