I hate to say this. I think you would be better off finding a new girlfriend..this "friendship" is not fair to you and she is lying if she says she doesn't have feelings for this person

Why the hell did you not dump this POS of a girl??

Why are you still with a cheater? Get yourself out of that relationship. Once a cheater always a cheater, and that's a FACT. Your gf has shown you once already... that she would cheat on you. You don't think she will do it again?

Get rid of her. Last christmas my girlfriend cheated on me and then fucked off leaving me alone for a very miserable xmas and new year. I let her back (actually I asked her to come back home) and she carried on eventually stripping my house bare and running off with all my money.

She's taking you for a ride and manipulating you just like I was. I was living in a dream world where you think people care because you want them to and conveniently ignore the evidence otherwise. It's not worth it.

wait? Are you male or female? Because if your girlfriend is bisexual and polyamorous maybe she needs a woman in her life to feel complete? My girlfriend is bi and polyamorous, she wants a relationship with a girl. She dates women and fucks other women. She comes home happy and she's got more libido it's great. We used to have less sex before I consented to her seeing women. I do get paranoid thoughts but I just make sure to voice my concerns so we can talk about boundaries and limits. Maybe you should just try to be happy for your girlfriend and consent to her having a relationship with a girl? 

If you choose to do that, I think you should tell her you want to remain her primary partner and that you want to spend lots of time with her but that you are willing to share time. Maybe down the line you will become friends with her girl friend and you can all spend time together.

 Denying her who she is will only push her away from you but if you accept her she will love you even more for 

I just want my girlfriend to not love someone else.