Being asked to let go of their anger and resentment feels like being asked to give up yet one more thing they have a right to. Even if they see how things would be better if they were less angry, they can't get past feeling like they shouldn't have to be the one who needs to change.
Zen, this is exactly the way I felt when I had to make a choice about anger. I actually said, out loud, "If I am not angry, how will I live? If I'm not ME, who will I be?"
I honestly felt I would cease to exist if I made a change.
I truly get how difficult it is.