how to forget his past.

Been with my bf for almost two years. Him and his ex broke up two years ago. She cheated on him and he cheated on her as well with me. She begged him back and he still picked me. I know they say once a cheater always a cheater but this man has not even texted or messaged a girl while dating me. He’s literally the definition of obsessed with me he has over 2,000 pictures of me in his phone and constantly checks up on me. But I can’t forget his ex. She left us alone but here’s the thing he told me he didn’t want engagement or babies with me and their sex life sucks and she just laid there. And I call him and liar and he says why would he be insulting himself saying he didn’t please her and knocking himself down etc. I think it’s a lie because my boyfriend is at least 7 inches so I don’t believe him saying their sex life sucked. he tells me if i believe him and stop mentioning her everyday our relationship will be better because we have some really bad fights because of his past. I get it is his past but that doesn’t mean lie to me about it. He tells me they never talked about kids yet he was finishing in her, and looked up fertility issues on his phone which he said was her and she thought she couldn’t have kids so she never mentioned having a baby together but they dated for three years. lived together and had everything together. Me and him just recently moved in and i can’t get his past off my head it’s like my gut is like what if he’s lying every 2 minutes bad thoughts cross my head I feel like i’m losing my mind because I can’t take the fighting. Advice?