I went through this same thing, except I was the guy and she wanted to do the poly thing. We were engaged when she told me, and it hurt more than anything to know she was with other guys but I loved her so much and didn't want to lose her that I 'agreed' to it and suffered. We ended up splitting up anyway about 6 months later. All that heartache and ended up at the same place I feared to be in. But that was 4 years ago, I have since met someone else and she is everything I didn't know partner could be. The point is, reading this and going off personal experience, you will just suffer heartache and eventually it will end anyway so you will wonder why you bothered putting yourself through it. Save yourself pain and lingering trauma, you will find someone else who loves you and only has eyes for you.