I feel jealous and resentful of my friend, what should I do?

One of my best friends, who is also my roommate, has really been on my nerves lately. It started a few weeks ago. There is the guy that I have been hanging out with some, and we like each other. It's not serious at this point, but I do really like him. Ever since my roommate broke up with her boyfriend in February, she has been flirting with every guy who will give her the time of day, and I understand that. She admittedly, "Uses guys for attention.' One night we all went out, and she kept flirting with the guy that I've been seeing. And I didn't care until he told one of our other friends that he liked my roommate. She is not the least bit interested in him (She says her standards are much higher than that), but that hasn't stopped her from flirting with him even though she knows it bothers me. Since then he has continued showing interest in me, but now I am uncomfortable hanging out with them both at once. And the worst part is that I feel really jealous and resentful of her because of it. Suddenly, all of the little things that annoyed me about her before are like major thorns in my side. And she's so pretty, she could have anyone she wanted. I have never been insecure, infact I think that I am attractive, so I don't know why this bothers me so much.I know that it is wrong and irrational for me to feel this way, and I feel guilty about it. I haven't expressed any of it to her, because I know its not fair to her. How do I stop feeling this way? I mean, we live together and are close friends, and I don't want something like this to come between us. On the other hand, I really like this guy and this is the first time in a long time that anyone has really liked me. What should I do?