Hmm...
Ok, I'm going to state the obvious here. Please don't feel patronised, because that's not what I'm trying to do. There are billions of people in the world, and about 50% of them are female. Wherever your boyfriend goes in life, he will come into contact with other women. Many of them will be in his age range, and many of them will be attractive. You are never going to be the only girl in his life.
However, you are his girlfriend. He wouldn't be with you if he didn't want you. Trust me, if a guy decides he doesn't want to be in a relationship any more, he generally doesn't waste any time getting out of it. Yes, some guys cheat, and if you've had personal experience of this then it's hardly surprising that you're insecure. But there are also many guys who don't cheat, and by the law of averages, not every man you have a relationship with will cheat on you. So just trust your boyfriend. If you ever feel jealous of another girl who is in his life, like his flatmate, try making friends with her. If you become friends with her, the chances are she would feel too guilty for doing anything with your boyfriend.
My male flatmate is very fit, but he has a girlfriend of four years. Before she came to visit, I was pretty jealous, and thinking, "Oh, he's just started uni. They're bound to split up soon!"
Now I've met her, because she stayed with us last weekend, and even if he was prepared to cheat on her, it wouldn't be with me, because it would be cheating on someone I met and liked a lot, and my conscience wouldn't allow it.