I'm a very jealous person! I feel sick when a girl in revealing clothing even walks past my fiance.?

I'm 20 years old and I know I am a very pretty girl and my fiance loves the way I look and he's very attracted to me. But I just can't help being sooo jealous because I just love him so much. I just think that he'll look at prettier girls and think he'd rather be with them than me...I always bring up his past and get upset and imagine how he was like with them, and I just need the constant reassurance that I'm better than his past and he wouldn't think of being with anyone else. As much as he loves me he doesn't like the constant nagging and bringing up of the past. I'm such a kind person with so much love in my heart for him and I'm very emotional, but I really want to stop being so jealous and move on and not have to say 'ohh I bet you looked at her and fancy her' every time a pretty girl walks past. Please help and give me advice anyone who has the same kind of problem!