Yours is a very tough situation and I struggle with advice. What makes it especially difficult is her unique ability to continue to string two guys along so well, without giving up sex. I'd have to believe she does have feelings for you or she'd give that up. Of course, she could be guy-like in her ability to maintain two sexual relationships.
One thing I do note about your situation is you both are acting from the position of fear. You can deny it if you wish, but I think you can't do tough love because you fear losing her. She won't run off because she to some degree fears losing you. I wouldn't be able to continue this relationship, knowing she still had this other thing going on. During my time away from my wife (divorced from her actually) I did realize that life could be fine without her. I could kick her to the curb if she cheated on me. Can you function fine without her?
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer
