FOL, I hope this works out ok for you, but I worry. I see that what you were doing was not working, in that Matt was not at all happy, and clearly things needed to change. But you have undergone such a radical shift in perspective that it is hard to believe that it will turn out to suit you.
You say "I am struggling to see the benefits" [to polyamory] and perhaps right now that is true, but as far as I can tell you were happy for the previous 12 years, and that sounds like a benefit to me. And I don't really understand how you seem to be invalidating that.
Yes, it had problems; yes, it could have been done a lot better. And maybe you really do need to focus on your marriage right now.
But I worry that because of the current problems, and perhaps out of guilt, you are going to try to shut yourself into a box where you will not fit.
So I guess I just hope that the recognition of some new aspects to the last years of your life that you were previously oblivious to doesn't cause you to totally deny or negate what you did experience.
And I apologize if I've misinterpreted, but this is how it has come across to me.