That's a tough one. First, welcome to polyamory.com
I don't think anyone has any words that can make it all better, I'm so sorry. All I have for you is to read, read, read, and give yourself time. Is she going away in a week? Or do you have some time to process this?
What have you read about how to 'tackle jealousy in open relationships'? Keep talking to her about it. Does she know you cry?
I'm hoping that my reply will bump this up and perhaps someone who has more experience than I can offer you something more. Read, read, read, there's tons of stories on these boards that sound so much like this one.
And know that it may not be for you. There are lots of people who believe people should be able to be openly gay, but they are not gay. You may believe in the ideals of poly, but you might be wired mono. It's not a horrible thing.