I am so sorry! Your story just sucks in the worst way.
There were a couple of things that struck me in your post.
First your belief that mistakes *you alone made* brought you to this place. Sweetie, you weren't in the marriage alone, but it certainly sounds like it. Like you had total responsibility for making it work. Your ex had just as much responsibility, and he did not do it. Have you ever heard the phrase that you can do everything right and still lose? When he says that you didn't do anything wrong, he is probably telling you the truth.
We are taught by our society that if we just try hard enough we will get what we want. We are taught by society that if we didn't get what we want, it is our fault. Much of the time this belief is totally inaccurate. This is one of those times. I don't know what the issues were in your marriage, but again, you were not in that marriage alone. You can't preserve a marriage the other partner does not want to preserve. You need to quit blaming yourself and second guessing the decisions you made. While I understand your distaste for poly, you could have said no, and he could just as easily have chosen to leave anyway. It happens in monogamous marriages all the time. Quit using your consent to be polyamorous as a club to beat yourself up for what happened.
Secondly, it sounds like your husband was not interested in maintaining his responsibilities to the marriage. I say this not because of the way that he treated you, but because he allowed you to move the children 2000 miles away from him. Yes, the reasoning sounds okay - but I know a guy who is getting divorced and he would follow his children to the ends of the earth. He just wanted away from the strife in his marriage that two marriage counselors were unable to fix - but not away from his responsibilities. It would kill this man to be away from his kids like that.
I am not trying to paint a picture of your ex as a horrible person, but that said, given his behavior is he really worth all your tears? I get that you feel very rejected. But, from my perspective, it is time you start rejecting him.
And I also would suggest therapy. While it is great to come and vent, I think you need more assistance than people on a forum can give.