It's part of the cycle Can. You go down and you come up again. He's angry because of the guilt and the shame. In the early days of the relationship they feel like they are cheating on the OP with their spouse. It isn't you. He's just in happy chemical land and you are a reminder of what a dirtball he is. Kids know more than we think they do. Focus on her and her needs. He will be gone again soon and it will be easier when you don't have to see him and feel his slights daily. You'll find a way to make it work.
Don't do the three Ps.
Don't Personalize this into thinking it's you or about you. You didn't cause it and can't fix it. Don't let it Pervade every area of your life. It is just your relationship with him that is off track. Don't view how you feel today as Permanent. It isn't. You will feel better.
