What do you do if you can't move on, and stop liking someone?

For some reason I can't put her out of my head, even though she has a boyfriend and is happy I still feel so worthless. I know were still friends but, I feel like I missed the opportunity. It takes a lot for me to get as close to someone as I have with this girl. I am thinking of asking her about polyamory, which I don't know is right or not. Basically I want to tell her she could have us both, I am satisfied if I'm just a guy on the side if it means we're something more than friends. I don't know what to do.