Why is it so hard to move on from infidelity?

About 6 months ago I was in an exclusive relationship with a guy for about 6 months. We weren't boyfriend and girlfriend yet, but we weren't supposed to be sleeping with other people...mainly because we weren't using protection. Long story short he was sleeping with a female who I knew to be a "buddy" of his. Since we weren't BF and GF I decided to give us another try. About 3 months into our official relationship he cheated again...with the same female. I decided to give us yet another try. 3 months later, things are going great. Well better than great. We've developed a friendship and ability to talk to each other that wasn't there before. He's dedicated to me now in a way that I know he's never been dedicated to anyone. So, the issue now is that he doesn't understand that I'm still hurt by what he did. Sometimes when we lie in bed and I begin to cry he yells at me for holding on the past instead of comforting me and telling me he loves me. He thinks that I want him to apologize when all I really want is for him to comfort me...that's it. A kiss on the forehead a smile, just so I know everything is going to be OK. He doesn't understand why I get sad sometimes or why things that I see or that he does takes me back into those dark memories. I'm doing my best to forgive him and move on, but he has the attitude that if I want to be with him I need to get over it. I try to tell him that the things I go through are not about him and that it's normal. After all it's only been 3 months. I've tried everything to forgive and forget, Prayer, research everything, but nothing is working. How do I forgive him? I think our relationship is worth holding on to. He thinks I'm being a baby...what do you all think?