Yep, I would stay clear away from her; it's wrong, as you know, and unhealthy for you. It sounds to me like she's messing you about, anyway.

I'm single and have been for a while, but I most definitely wouldn't sleep with a woman who's taken or be otherwise actively romantically involved with her; that's just wrong. VERY few exceptions. Talk about it, sure, make plans, sure, but I'd be smart until things are finally over. You should, IMO, always wait until the previous relationship is over, completely, before "moving in", as it were; if anything, think of it as courtesy for the other guy.

You recognise your error, so now you can take action to make things right, which, from the sounds of it, is to get away from her. The last thing you want is to deal with a very angry boyfriend who's looking for revenge, if you know what I mean.

Tell me about it, but that doesn't give you free reign to be messing about with someone else's girlfriend. Don't get me wrong, she's worse, if anything, for cheating on her boyfriend, and screwing you about. At the end of the day, it takes two to tango.

Best of luck getting out of that situation. As for the future, maybe avoid women who are taken?

PS
I do appreciate how hard it can be to be alone for so long, and when something comes along... I can appreciate the temptation, both sexual and romantic; I hope I can stay as stubborn as I've always been about these things.