My long term boyfriend cheated with his ex and now wants to rebuild the relationship?

We have been dating for over a year, and he cheated on me with his ex girlfriend and had been doing so for a few months. She showed me loads of emails and pictures that they took together, including messages of mine that he had been forwarding to her for some odd reason. I feel so betrayed, deceived and humiliated. She has been trying to ruin our relationship from day one, and I feel as though she is happy now that she has succeeded. The affair was not only physical, but emotional. In order to prove that to me, she called and put my bf on speakerphone, so that I could hear him tell her that he loved her. I was devastated. I felt as though she was intentionally trying to hurt my feelings. She showed me emails where he was talking about me behind my back, and convincing her that we were not together. She did not believe him and eventually came and asked me. He has since told her the truth, has begged for me to take him back and has promised me that he has changed. After this happened I stayed for a while, yet I am now realizing that I am having trouble forgiving what he did. Sometimes it just makes me look at him in disgust. I feel bad because he has been trying so hard, however is it possible for it to be too late to try ? If he really cares, why did he cheat on me with her in the first place? He constantly argues that this happened months ago, and therefore believes I need to at least try and move forward with him and give him a chance. I would have never lied to him this way. Our relationship seemed to be very healthy and we talked about everything (at least I thought we did) My previous boyfriend cheated on me, and he promised to help me get through it, and that he would never do the same to me. Obviously he lied about that :( Not to sound cruel but his ex has a very selfish personality and is also not very attractive. She just isn't a nice person and treated him very poorly. She manipulated him a lot and brought down his self esteem. I don't understand what caused this mess to happen all of a sudden ? And why he risked our relationship and everything that we had? His family loves me, my family loves him, and we were always inseparable. He was my best friend and although I despise what he did, I feel lonely and sad that the relationship ended this way. I know this sounds crazy to some but he really isn't a bad person. Other than this incident he has always been so kind to me and such a sweet guy. What gives? He keeps begging for me to give him a shot however I don't know that I can forgive him. The pain is too strong. However I still love and care about him a lot. I am naturally a very stubborn individual but I dont think I am being unreasonable here? Please help ! I would appreciate mature and straight forward answers.. thank you all for your input I truly appreciate it