Infidelity of this magnitude is hard. It's unfair, and terrible pain.
In your situation, the WH has treated you horribly for many years.
We have a saying around here, "When someone shows you who they are, believe them."
It's time for you to believe what you see. This man is not a friend to you or your child. He is NOT safe person for you and he has been abusing you for a long time.
Abusive, dysfunctional relationships are often more difficult to break free from.
Please read this book- "Why Does He do That?" by Lundy Bancroft. The author works with physically abusive men, however the psychology behind that and what you are experiencing is the same-- especially for the victim. This book has helped countless people save themselves.
It is time for to save yourself and you do that minute by minute, then hour by hour, then half day by half day, etc.
You do that by reaching out for help here at SI.
You do that by learning about how your own thinking became distorted, and why it became distorted. And then retraining your thinking.
You do that by small actions every day.
Someone I once loved gave me/ a box full of darkness/ It took me years to understand/ That this, too, was a gift. - Mary Oliver
Just for the record darling, not all positive changes feel positive in the beginning -S C Lourie