Found out he slept with someone else before we were official

So my boyfriend and I have been dating for about a year and a half now. Before that, we were just casual for a little less than a year. At the start of our casual hooking up situation, I established a boundary that I would want to know if he is seeing other people, whether it’s emotionally or physically. We also had a discussion about keeping things casual because it was what worked for the both of us at the time and what both of us wanted. During that year, I caught feelings for him but kept pushing it down because I knew a relationship might not be feasible considering our discussion and that he would be moving to a different state soon. I visited him over the summer before he moved which I thought would be the last time I see him, so I told him how I felt and also reassured that I wasn’t seeking anything out of it but just wanted him to know and move forward with my life. After that, while we were still in contact a little, I tried my best to focus on myself and it was great. He asked to visit me again in the fall before he moved and then I thought that would be the last time for sure. After he moved, he began to facetime me and talk to me much more and on a much deeper level. About a month into that, I had enough and asked what he wanted from me. I had told him I had feelings which till then, I never knew if he even felt the same way back. I was frustrated of being in this limbo and pretty much said not to contact me again because I told him where I stand and that I wanted to move on. He said he always liked me but was scared to admit it to himself especially because he was moving. He also said he wanted to do things right and we decided to give it a chance. He officially asked me out in person a couple of months later at the end of the year, during those months we were exclusive and spent time getting to know each other more. It was great, he did everything right and our relationship was great. However, fast forward to spring of the next year, I jokingly asked if he had seen anyone while we were casual. I wasn’t expecting the answer to be yes. While I was overthinking my feelings for him and turning others down, he wasn’t doing any of that. He hooked up with another girl twice. He told me he didn’t enjoy it the first time and was scared to say no the second time since he never reached back out to her. But that those made him also realize that he only wanted to see me. Honestly, hearing this was kind of soul crushing. It was so easy for me to say no to people because I had such strong feelings for him. I know that differs from person to person but at least I was true to my own boundaries. It’s been a bit over a year since I found out about that and I feel like it’s damaged how I view him and our relationship so much. Despite holding myself in high regard, I spend more time than I’d like thinking about him sleeping with the other girl. He has been there and open to talking about this as much as I want, but there has been a lot of ups and downs. What would you do in this situation?