Wiseacre- I know how you feel, pretty much (no situation can be exactly the same). I am thinking I am poly and my husband isn't sure if that is what I am or if it is just an excuse. He is also against the idea. He keeps telling me to put it off, then I found someone I really liked and he was faced with it again. It doesn't go away.
What most people will tell you is this: You have to make a choice. Is she worth you staying mono? Will she think you are worth being poly? Will you need to move on from her and find someone who is more into your line of thinking?
The answers to those Qs for me are: My husband and I decided to never divorce. We are worth staying together, but we just do not know how to compromise yet, so we are taking it slow (snails pace), having talks, just getting him used to the idea. Not letting him forget it like last time, letting him know I think of it often and feel this is something about me, not him or us. Perhaps that could help you?
Only you can decide if she is worth the sacrifice and only she can decide if you are worth the sacrifice. Know that she is feeling hurt right now, as annoying as it can be when people are as open about love as us, she just needs time and love. She may never warm up to the idea, but give her the chance.