Wow....great post, redpepper!
Bunny, you asked, "how can he help me?" The main thing he can do is try to make you feel loved. I am in the same situation as you - trying to accept my husband being poly. I did feel less loved at first, but in the course of our working things out, I've come to feel MORE loved, and that has made all the difference. Without that, I could never have opened up to this possibility.
The other thing that has helped is becoming friends with his girlfriend. That way I feel like my life is enriched too, not just his.
I also have a very similar background to you in terms of relationship experience and insecurity. Part of what made this so hard in the beginning was that it opened up all my old insecurities. It also made me realize that I was much too emotionally dependent on my husband, and that I need to find love and happiness in other places - primarily, in myself. It sounds like you need to do some of this inner work, too.