How do I get over the fact that my boyfriend cheated on me?

Okay well hers is the thing, my boyfriend cheated on me a long time ago with his ex girlfriend.He admitted it was a stupid mistake and that he loved me. After he told me what he did the next day I broke up with him right there and then. I didn't want to talk to him or look at him or anything. The summer came and we both did Cross Country. I thought I was over him, but I couldn't help falling for him again. Slowly I started talking to him again and I made it clear I just wanted to be friends. Then the school year started and we talked more and we finally went out again. I told him that if he ever anything like that again I would never talk to him again. That I would act like he didn't even exist. Ever since then he's been the most perfect boyfriend. I know he was sorry, and HONESTLY sorry. He always does little things for me. I mean the guy is so great he even gets me orange juice when he thinks I'm getting sick! Well anyway, I always tell myself to put everything that happened behind but I just can't. I still have that sense of Betrayal. He doesn't even talk to his Ex after what happened or gives me a reason to doubt him one bit. I know he would NEVER do that again. He showed me that I can trust him but I just can't get over it. We have been together for 2 years and I love him.. So how do I get over the fact that he cheated on me even though it was so long ago?