So you met someone that was incompatible. You became involved with this person under the condition that he changed in order to be more compatible. In the end, you were still incompatible.
Damn, that sucks.
It's a good thing that life goes on!
How I've phrased things might seem to be a bit abrasive, but it certainly puts a new perspective on things when you break them down into the sum of their parts.
Any judgements I make on what this guy's possible motivations were is pure speculation. I don't know the guy, and really, the only person that really knows a person's motivations is that person.
From what it sounds like to me, is the guy knew that poly was a limit for you, and still wanted to be with you. Maybe he thought he could change, maybe he wanted to for himself, but from the sound of it, he wanted to for you. Or, yet another option is he thought that once you were attached you would be willing to give it a try.
Whatever the case, the guy was at least honest with you. He told you that he was poly from the get go, he gave being monogamous a sporting try, when he was unable to, he was honest with you. When you asserted that you couldn't bend, since he couldn't either, he ended it, which is way better than trying the patch a sinking ship with bubble gum and going down with it anyway in the end.
As the ever-wise "they" say, "some things just aren't meant to be".
I still hear you scream... in every breath, every single motion