What should I do with this girl who dates other guys?

I've been talking to this girl for about 6 months. She told me from the start, she doesn't want anything, she had just broken up with her B/F. When she said that, I kind of thought, then why are you talking to me? She asked me what I wanted, it sounded like she wanted a f*ck buddy or something but I just said "I dunno".

At first, I was like whatever but after a while I really became attracted to this girl. Fast forward 6 months, we are now roommates and live in a house together. We are like a couple but not, f*ck buddies I guess. We're together a lot and show each other affection. My attraction has grown even more but now shes dating other guys... I get really pissed off but can't do anything about it. I act like its alright with me, but it disgusts me, I don't even wanna talk to her after thinking about her with other guys. I feel like one day shes gonna end up kissing or f*cking one of these guys.

Anyways, what should I do? I think my best bet is to get it over with, I'm only hurting myself, because I'm the one who is attached. I also feel like a little b*tch because I do a lot for her but here she is, dating other guys, maybe something else...