All I have to add is that your girlfriend may very well be poly but likely doesn't have the relationship skills at this point to be able to manage more than one relationship. She is only 17! It sounds to me like she still has a lot of work to do on understanding herself and what she really wants.
You said in your last post that everyone is telling you that you should leave her. However you didn't say if that's what you want to do or not. Do you think she's willing to do the work required to have a more mature view of herself and her relationships? Are you willing to hang in there with her while she does the work?
Everything will be ok in the end. If it's not ok it's not the end.