Arguing constantly

We have been together going on 2 years but have known one another for quite a while. In the past he has cheated on me and I decided to forgive him. I know I love him but I really can't get over the fact that he really cheated on me. Don't get me wrong. He has changed a lot. He has proved a lot. But the nightmares, is the worst. Sometimes when I mention my nightmares and ask are we okay he gets upset. I feel that if anyone I should be able to talk to him but he is not easy to talk to. Idk what to do. This is getting the best of me and I feel like giving up sometimes!