I was reading your initial post where you said that you had a conversation with her about not doing anything that would possibly hurt each other. Were the two of you specific in what that meant to both of you? There has been a lot of discussion on boundaries on here and one of the things that comes up frequently is that one person's definition of what crosses a boundary is not always the same as another person's.
I also saw you say that you had hoped that she had moved past being poly. That's not how it works (usually). She may agree to move slowly with anyone else in her life in respect for your feelings but chances are if you tell her that she must be monogamous with you forever she may very well fall in love with someone else and hide it from you.
Everything will be ok in the end. If it's not ok it's not the end.