get over the past

ok i really wish i could forget the past because it hurts so much. me and my boyfriend have been together fro a year and a half. but last year he broke up with me. the worst part he went back to his ex. he was with her for a lil while and then got back with me. the problem is they did a lot of things with eachother that i thought he would of waited to do if he really loved. now everyday i think bout it. like if we are kissing or just laying next to eachother i start to think they did the same thing. it sucks cause i wanna get over it. me and him have been through so much together but its still hard. i want to marry him and have kids and just live the rest of my life with him. but will i be able to do that if i cant forget the past? i need advice please.