Before I start, I'm going to give a little information that you may want to know.
I'm 19, female and have a boyfriend whose age 25. (Age isn't the issue, ignore the age difference). We are online dating and have been with each other for 1 year and a few months)

I get really really upset about his past. He been in 4+ relationships with other girls and out of those relationships, he had sex with 3 of them.
All 4 cheated on him and lied to him. He was looking for love and had sex with them too early. He had sex with one of them in 5 days after meeting her. This hurts so much. Why have sex with someone you don't know so much about? He said that he trusted her and all that, so they started making out and soon got into each others pants. This also hurts IMAGINING him ROLLING around in BED, kissing, touching each other.. :'(

All the girls he had sex with weren't virgins. They took his virginity. He haven't took any other girls virginity. I am a virgin. I got kind of forced to being "sexual" by a 11 year old boy when I was only 7 years old. He tried to have "sex" with me, but he only put his 1/2 inch p*nis inside, no moving or anything, just there. So, I consider myself a virgin.. My boyfriend is upset about my past, but I don't think its as bad as his..

Just thinking and imagining of him being with some other girl, in bed, having sex, hurts so much!

I feel disgusted. I have searched this issue that I have online, and people respond, "Past is the past" "Stop being immature". But that doesn't help. I know the past is the past, but he did it and he had sex with those women..

Maybe I'm upset about it since I haven't physically met him yet?

My whole point in this question is HOW do I STOP remembering his past? It plays in my mind like a movie.. I want it to stop.. Everyday we have arguments about this issue and I want it to stop. Please, anyone.. please help me to forget his past. He loves me so much, and he's dealing with this because of me, but I can't stop thinking about it. He said I'm the perfect girl he ever had and that he loves me more than the other girls he's been with. He said I understand him, I can be trusted, that I'm very funny, have a good personality, etc.

Maybe I'm upset because I have a weight problem? Maybe I'm upset that the girls are prettier than me? He said I'm more beautiful than them, but I don't seem to believe that..
Please help and please be kind to me. I don't want to be offended or be more upset due to mean comments. Thanks for your help. :(