It sounds like you can eithr have pain now or pain in the future. I have noticed that since monogamy is assumed, it is more likely for someone to go from monogamy to polyamory. It is rare to go the opposite. (Though it does happen.) It sounds like you have tried to make it work so she could be comfortable with the idea without putting too much pressure. However, it sounds like she doesn't like the long term concept.
All i can suggest is that you can try to get her to check out polyamory with an open mind. Maybe chat on a forum or find a good book. But if she thinks she will be unhappy, it may be best if it goes no further than it has. You are not limiting your love. She is just drawing a boundary and limiting hers.