Code red. She was predicting doom from day one, and she thought she was basically going to steal your boyfriend from you? (The walking away with your boyfriend reference.) It sounds like she had ulterior motives in the form of ending up with your boyfriend, and she had to go along with being into you to be with him. You might care for her, but why did she even get into the relationship if she was not really into it? This is a case of her being in over her head x 10. This child--yes child--is not equipped to handle a relationship of this magnitude. It is not about her. The relationships at hand are:
you + your boyfriend
you + your girlfriend
your girlfriend + your boyfriend
you + your girlfriend + your boyfriend.
If this is her first real relationship, she is stuck on dynamic #2 and forgetting about the other ones that are equally important. I am not even that old, but in my eyes, she is child, still has milk on her breath, and is wet behind the ears. She has no life experience and no experience with relationships, and it is apparent. What on earth made her think she was ready to be in a poly relationship without even having one successful relationship under her belt?
She sees that she is causing problems between you and your boyfriend. She knows what she is doing. And your boyfriend is way out of line when he says he will leave you both if you leave. That should tell you how much he "loves" you right there. Next time he says it, leave, look over your shoulder, and tell him "bye." Walk out and shrug it off. You are 22, and it may seem like he was meant to be the one or what have you. Sweetie, a real man does not say things like that. He is willing to leave you heartbroken because you are tired of dealing with her immaturity and you want out? I say let your boyfriend go and be with her. If you are just determined to stay with him, break up with her. She does not act like she wants to be with you any way. She can transition from girlfriend to your metamour and be one leg of the V.
She needs to move out of your home. I refuse to be uncomfortable in my home. I refuse to have drama, so I stop it before it even enters the front door. My home is my sanctuary. If your boyfriend wants to follow her when--not if--she moves out, let him go. He can deal with her inconsiderate, immature, childish behaviour.
-Ry