I am sorry that it sounds like this was sort of... foisted on you then?
I feel like I didn't sign up for this and that it's been put upon me and I struggle with keeping it going; I often feel like I don't want it in my life but I also want to give to him and allow him to be his real self.
Then does that mean you want to split up? Is that your current struggle? The "should I stay or should I go?"
So he can have this and allow him to be his real poly self and be a free dating agent? And you can get what you want here and not have it in your life?
I'm not clear what kind of support you are looking for, but wish you well anyway.