Go with your heart. Open relationships can get very ugly. If you are not comfortable with her being with others, (and I sure wouldn't be!) then don't do the open relationship just to try and keep her. It sounds to me like you may have already lost her. My ex cheated on me and she wanted an open relationship. That's not my thing so I left her. There was no way we could be together after she cheated on me, I just couldn't trust her. And there was no way I was having an open relationship. I value myself and believe I deserve a one woman woman. Best of luck to you.