How can I deal with my jealousy and my boyfriend's previous sexual past?

My boyfriend and I have been together for 10 months, and we are both 18. I have never even been on a date before him, he's my first everything. However, he has had a girlfriend of 2 years before (but that or she doesn't bother me.) What bothers me is that after his girlfriend, he was the type of guy who slept around with "trashy" girls and had things with girls left right and centre. And they were all sexual reasons only. Anyway, I trust him completely and I know I have changed him, and our relationship is very strong and mature for our age. But what I can't deal with, is this one girl he had a sexual thing with, I have to see her every single day because we sit in the same class next to each other (seating plan lol.) She is known as our "school slut" and has been with 40 people apparnetly, however the difference is, she actually had feelings for my boyfriend, but he ended it with her. Everytime I see her, I am just reminded of the fact he has been with her and what they have done and I can't deal with it. I just start seeing them together in my mind and thinking of what they have done etc and it ruins my day. I can't control it. Also, this other girl he had a thing with about a year ago was invited to his recent birthday party (because his friend wanted her there to "get in") and that ruined my mood because I was jumping to conclusions and I still dont understand why she had to be there. Maybe its just me, but I think you shouldn't talk or be associated with people you used to like or have feelings for once you are in a relationship. I'm just always scared that when he sees these girls, he will think about the times he was with them and maybe compare them to me? I dont even want to invite him to my school formal because one of those girls will be there and I can't stand the fact if they even make eye contact. Please help me I know I am being so unreasonable, but please help me, how do i deal with his sexual past and deal with seeing these girls so often and also how to deal with the fact he still sees them too at parties or school etc?
How can I deal with my jealousy and my boyfriend's previous sexual past?
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