Excuse me, why do you have to forgive him??
Millgals (is there more than one of you?) I'll answer that. It's only natural to resent, hate, bear a grudge or (fill in the blank with the word of your choice) a spouse who has cheated and mistreated you. Here's a husband who's left his cancer stricken wife for another woman on their son's birthday and it doesn't get much worse than that and she has every right to feel hatred, resentment and etc.
The problem with this (and bnt knows this) is that as long as she holds onto those emotions this b*stard has a grip on her life and will continue to do so until she can forgive him. Once she is able to do that, he won't matter any more and she can move on with her life. The only thing she will have in common with him is that she is the mother of his child and that's standard stuff for all divorced parents.
Here's another thought, bnt is considering a new relationship but to succeed she first has to end the old relationship. Have you ever met someone so hung up on past injustices that it controlled their current relationships? I have, and now matter how justified, it gets really boring, really fast, and bnt knows this. "how to forgive my husband and remain friends." bnt; since you already know this is the right thing to do counselling may help get uyou there but time and distance are your best friends. Have contact only where absolutely necessary and focus on your health and the future. I know, it sounds easy but I'm unaware of any magic pills to cure your condition.