How to overcome being cheated on..?

I been in a relationship with my boyfriend for 4 years. He is my high school sweet heart. He is my first love. A year ago he cheated on me and had another relationship for 5 months. His mother even knew and i was close to his mom. when i found out, it crushed me to pieces. Since he was my first and only person i slept with. IT made me feel like i wasn't good enough.. made me question myself A lot.. like what did i do for him to cheat on me.. he said he was sorry and he would never do it again. Now he changed a lot for me .. He gave up his bad friends and changed his number so that girl wouldnt txt him again. But now anytime i see a movie that shows cheating i re-live everything.. I found out on my Birthday that he cheated on me... Its almost my bday now and im already living the pain.. i want it to work and i know hes changed. But will i ever feel the same about him again? When he tells me that he going to work, a bad thought comes to mind. I think is he really? During the whole time he cheated on me he lied so many times that now its hard for me.. not only to trust him but everyone. Sometimes i even have vivid dreams about it.. Will this feeling go away by time? i wanna work things out with him..