Goodness, i don't know what to say really and i'm confused, your a girl aren't you? So if your girfriends up to something with this Bill bloke then it looks like she might like both men and women?

But still, even if she does then that's still no excuse for cheating on you, and for her to actually admit that she "may"end up sleeping with him, well i'm gobsmacked. She's certainly got some nerve hasn't she?, if it was me i'd pack her gear up in a set of matching black bin bags and change the locks while she's out. Seriously you can do so much better than this and if this is what she's like after 4 months which is the time when you're still normally on your best behaviour in a relationship then what on earth is she going to get up to in years to come?

I'm not surprised you are feeling depressed and that your emet is playing up with all of this stress and upsetment going on in your life. If it's not practical for her to leave and get another flatmate, is there somewhere else you could go or are you both tied up into the both of you having to stay there until the lease is up.

I wish there was something more useful i could say, but put yourself first and don't let her walk all over you.