First of all, welcome to the forum and *big hugs*. What you're grappling with is by no means easy, and while I don't really have much to add that hasn't already been said and probably better than I could say it, I wanted to offer my support and comfort.
In general, I do think it's awfully big of people to consider polyamory as an option after a breech in trust, and I commend you for trying to be open to possibilities, but truth be told, opening a relationship that has unresolved trust issues from past trangressions and isn't strongly rooted in open and honest communication isn't going to solve any problems... it's more likely going to create more problems, and more resentment and more heartache.
My heart goes out to you. I know I'm merely a stranger on the Internet, but I live in the same general area as you, so if you feel you need a friend feel free to PM me here on the forum and we can either chat online or go out for tea or something.