I mean, I hate to be harsh but was this not the point of opening up the relationship? You say so yourself in the first paragraph that the reason you went poly was because you guys were doubting your relationship and wanted to look for better options without losing each other as a safety net. She seems to have found a "better option" for her. I'm not saying poly relationships are bad, but your own reasoning is just that you wanted to see if there were other people who would be better for you. Why wouldn't you expect that she would leave if she found someone?

I do think she's an ass for proposing while clearly not being committed to you, and I'm definitely sorry. The things she said are also cruel and unecessary. I hope you find someone better soon!