Dont dwell on the past. Dont try to blame children's behavior on the divorce. Dont think of ways to fix your ex spouse to your liking. Dont wish your ex spouse will disappear. Dont get into positions where you feel overwhelmed. Dont let little things get you down.
Your XH wants to help. You need to figure out for yourself how much you are willing to let him help. I think it is a great situation for the kids if two divorced parents can work together as one for enforcing certain rules and discipline for their children.