I'm sorry you're hurting right now. It is one of the unfortunate facts that most relationships end. Poly can be both a blessing and a curse- it can give us more opportunities for relationships and connections, but then there are more endings as well. This time you got the short end of the stick, absolutely, but it seems like the issue was more her NOT being able to be poly than poly itself. People choose one person over another all the time in mono situations- usually a benefit of poly is not HAVING to choose.
As Kaylee says to Mal:
You just gotta have faith in people.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack