You need a poly friendly counselor stat, brother. Because right now you threatened her inadvertantly by wanting to stop the poly (aka end her other relationship). I know your intentions are good, but her opinion of what you suggest will be colored. Don't let it offend you. If you really want to save it, compromise....you'll agree not to shut her down if she agrees to see q counselor with you and work on realizing that her feelings are not competative....you don't have to be in the doghouse because she likes this guy and is having sex with him. But, if there's something wrong, she can't just ignore you and hope it goes away by running off to the NRE fantasy land. You're adult, you have to be able to do both, and its not easy.
Go to a counselor, talk. Make sure she knows you are worried about you two....its got nothing to do with guy. But it does hurt when it feels like she's not addressing her responsibility to your relationship
Appologize (she may feel threatened by needing a counselor)
I hope she decides to work as hard as you seem to be. That's what it takes sometimes.
Good luck brother.
“For an instant I think I saw. I saw the loneliness of man as a gigantic wave which had been frozen in front of me, held back by the invisible wall of a metaphor.”
― Carlos Castaneda, Journey to Ixtlan