I have been with my boyfriend for almost 5 years, with a 3 month on and off break in between those years (2 years ago). I trusted him to the fullest without a doubt and then during our break he started to hang out with another girl (an ex co-worker). There is speculation that he had a sexual encounter or multiple sexual encounters with her. She claims they did (she called me when we were back together shortly after the break), He says nothing happened. I do not know who to believe and it haunts me until this day. It has changed the dynamic of our relationship. I have no trust in him and it has made me insecure in the relationship. He's in school, different school than me, and is in a child development major so he is surrounded by different girls all the time. He has classmates with the same name as the ex co-worker and the classmate randomly texts him with questions. It doesn't help. I really do want to be with him, but I'm just so sad all the time about what happened still and I get upset over little things that have to do with girls. For example, he has another classmate (K), and I saw a random text from her and invited him to play this game. I even asked him about the random text to see what it was about and he explained they were talking about the phone games. I then found out that he plays with her in every single online game that he has (wwf, hwf, swf, and mini golf, and maybe songpop). I know in my mind I shouldn't be upset. But I can't help how I feel, and it sucks so bad, because I just feel like I'm ruining my relationship getting mad over trivial things like these. I tried to tell him that this stems from what happened in the past, but he doesn't want to hear it. He just says you have to learn to trust me, and we should focus on the future and present.

He used to be insecure in our relationship because I had a couple of good guy friends, and so I tried to communicate my feelings thinking maybe he would be supportive or at least understand where I am coming from, but I feel he just dismissed my feelings.

Any tips to cope? Or advice on what to do?