Once you are OUT the door, all you have to say is that you're unhappy in the marriage and, therefore, are moving on. If he tries to cajole or persuade you to come back, that this time he's really going to change, if he tries to guilt-trip you or tear you down, pay no attention. It's a ploy, whatever he says. There is nothing to "discuss"--you've HAD the discussions, remember?
"I'm done." That's really all there is to say. All it takes is for you to DROP the rope.