I'm very sorry for the difficult time you are having. I don't have children, but hopefully some of the people here that do can post some better advice on how to deal with this situation and kids involved.
What you are doing is showing your child what true love is, what patience, is, what commitment is. You are a fine example of a wife and mother. Don't put yourself down for DB'ing. You're standing for what you believe in and that's a beautiful lesson for your child.
Hugs.
Most of us really marry only once. First love endures, even unto our dying day. And we never really divorce.