He wanted another woman

so i have a problem in my relationship that i don't know how to resolve by myself, so i am asking you guys to help me, if you can. my boyfriend and i are together for a year now. a fiew months ago, just when we really got to know each other and everything was great between us, i discovered that he was having some kind of online sex (no cameras) with his ex, actually they were together for like a week 4 years ago. i almost dumped him, but he convinced me that he loves me and that he is sorry and will never do this again. (he said he did this with her because it was something new and interesting, because we never did this together). i know he means that, i saw what they were talking about, he told her how great i am and he actually told her that he will not do this with her anymore because he feels like he is cheating (he wrote that to her a fiew days before i saw these conversations). she also wanted to have sex with him and he said no, because of me, but he wanted to do it A LOT. so i am not really angry at him for this whole thing. the real problem is that, even if now everything is ok, he tells me the truth about everything, at least i hope so, and he is absolutely amazing, every time we get intimate, i think about how she turned him on and how much he wanted to have sex with her. i think about that all the time and i am getting depressed. i am nervous all the time, i review all of this over and over again in my head. i think it's just because i am scared that he wants someone else more than me and that i am not so interesting to him anymore, or i will soon not be. as i said, everything is great, the sex is amazing, i know he truly cares about me, we do what he used to do with her, and it turns us on very much. he says he wants to spend forever with me. but i still can't get all of that out of my head and keep thinking how much he wanted another woman (he really did according to what he wrote). i love him too much to leave him, but i can't continue like this. i know it maybe doesn't sound like a big problem, but it is for me. can anyone tell me what you think? what should i do? what would you do? i apologize for my bad english and thank you for your help!