Can't let go of the past

So, my boyfriend and I have been together for 2 years. Things are amazing at the moment. I feel very much in love, he's great, very affectionate etc. I want to think of a future with this guy, but there are several things that happened in the past that are holding me back. When we first met, he was still with his last girlfriend, something he didn't tell me about for several months. He did break up with her 2 weeks after we met, but until that moment, he was cheating on her with me. I also know he had cheated on her a few years before that too. At first, we weren't very serious. We both got out of longterm relationships and didn't want to rush into things. I suspected that he was seeing other girls, but I never asked. Things started getting more serious however. I fell in love and got more emotionally invested. This is where it got complicated. We talked about our own views of where this was going, and he was very clear about the fact that he didn't want to be monogamous. Although this really hurt me, I acted okay, because I was afraid to lose him if I was honest. I know, this was a big mistake. I should have been honest about how I felt at that moment. So we had an open relationship. The more I got confronted with his behaviour though (finding other girls' underwear in his bedroom etc.) the harder it got to tolerate. A few months later, I decided to be honest after all and I told him I really couldn't handle that he was seeing other girls, which surprised him. He repeated that he didn't want to be monogamous. I have to mention though, I was just about to leave the country for 3 months, so I was understanding , even though it was hurtful. I told him that I would try, but that he had to make sure that I would never be confronted with it, because that would be more than I could handle. So, 3 months later I came home and we took off where we left off, but I noticed that I felt more and more insecure around him, I would get sad and jealous whenever he texted another girl or mentioned anyone, even if they were just friends. Eventually, about a year in the relationship, he told me that he was ready to really commit to me. I was really happy, obviously, and I thought that all of the drama and sadness would finally be over. I was wrong. It's been almost a year now, and the memories of those times haunt me. Even though he has promissed me to be exclusive, I still get jealous all the time, I have nightmares about him cheating on me and I feel that my self esteem has dropped dramatically. I don't think I've really forgiven him for causing me this much pain. I want to move forward with him, but how do I let go of the past?

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