Advice pleeeaase!!!!

1.5 years ago my boyfriend cheated on me. He says they never actually had sex (don't know if that's true or not) but there was definitely emotional cheating, sneaking around together, kissing, I don't know what else they did and I don't want to. He lied and treated me like garbage the whole time this was going on. I ended up finding out about it, mainly through the girl, although neither of their stories match up. He said the reason he did it was he just "wasn't thinking". That could have maybe been an acceptable excuse, but this went on for at least 2 months! You don't just "not be thinking" for 2 months. He knew exactly what he was doing. He also said she manipulated him into it. Anyway, I agreed to give him another chance and try to forgive him. We've been together for almost 3 years now and he's been perfect ever since, he hasn't stepped out of line at all (I know that 100%) and he's very in love with me. The problem is I'm still having a really hard time with it. I think about it every single day and it's the last thing on my mind every night before I fall asleep. I snap at him so easily because I'm still so torn up about that deep down and I hate it because I am NOT an angry person at all, but that just makes me so mad and upset. I feel suspicious of him even though I know he hasn't done anything and feel jealous of other girls who have guys who didn't cheat on them. It feels like the only fix would be to go back in time and undo it and that's not gonna happen. He says he wants to marry me but I just resent him so much. I try not to bring it up often because he's trying so hard for me but it doesn't take the hurt and feeling of betrayal away. One of the things that attracted me to him the most when I met him was that I could trust him and he was different from other guys. But this made me realize he's really not different and now one of the big reasons I loved him is gone. I feel like such a bitch for being angry over something that happened a year and a half ago but it's got to the point where even discussing cheating just kills me. I'm just way too sensitive about it. I don't know what to do, help me ladies! I feel like I want out of this to start over but I can't. I'm 19, why am I even trying so hard?