How do I deal with my insecurities?

I've always been very insecure in my past relationships, and it's been a huge problem for me. My main issue is I become intimidated by girls who I think are better than me or more attractive. For some reason, a girl in my sorority has gotten under my skin this past year. She's not very pretty but I've heard about her sleeping around and I've seen her always dressing slutty to get attention. In truth I think I'm insane for feeling any envy because she doesn't seem to have anything going for her other than her body. Shes a senior with no plans after college, she was on academic probation, and frankly she's a bitch. She is however a very flirtatious person and a lot of guys are always saying she's hot and they would fuck her. I've always been a tiny bit jealous of her because she received a lot of attention from guys and was super popular, but that was more of an admiration of her confidence. But then it got bad when I was at my formal with my SO of 5 months and I saw him watch her walk by in a low cut dress wearing a push-up bra and he said "she's cool". He said this because apparently when he's smoking with his frat brothers, she and some other girls from my sorority smoke with them and hang out. This stuck with me and kept me up at night thinking about how my boyfriend probably thinks she's hotter than me, and maybe that's true but if I think too hard about it I go crazy. The jealousy subsided later on until I went to visit my boyfriend at our university and while we were getting lunch she walked in wearing nothing but a thong bikini. And to make matters worse my boyfriend told me he smoking with friends the other day and she was there. Now I just picture them hanging out talking while she's dressed slutty and wondering if my boyfriend wants to fuck her. I know I sound jealous and crazy, i realize that if my boyfriend wanted anyone else he wouldn't be dating me. But she's just always so present and she seems to be everywhere reminding me of my insecurities. I want to know how to just ignore it. If there's anyone who's dealt with having a lack of confidence in relationships before, tell me the secret to ignoring all the sluts and just having trust in your SO.