First of all, I am so sorry for you. What your husband did was wrong. Cheating isn't okay no matter how poly you feel yourself to be or how much you love the person you cheated with.
I have one question; when your husband told you he might be poly, did you just leave it at that? Did it ever come up again before his betrayal was uncovered? Or did you think it should be enough for your husband to be just friends with this woman? It's likely that after the discussion, you had somewhat different understanding of what you had agreed on. Or it might be that you had a perfect agreement, and he chose to go against that.
What I would do in a situation like that is take a lot of me time. You are under no obligation to hear him rant on about his other love. Since you have kids, it might be hard to consider a temporary separation, but if you don't feel like talking about it, you can just say that out loud. Get into therapy and suggest he goes separately, so you don't have give therapy to each other.
Me: bi female in my twenties