sucker punched 

I feel for you, and I am a guy! And I feel for your husband. He is confused and scared, of that you can be sure. Men have an incredible burden placed on them in our society. Just stop and think of what we have to hold together. And we aren't allowed to show emotion. That is a still a sign of weakness for men in our culture. Many of us get to our forties and feel completely unfulfilled and disappointed about where our life has led. Especially if we got married early. If there was no infidelity perhaps this marriage can be saved. As Ken said, find out the core reason he is angry. Plead with him to go to counseling and talk about it. Even tell him a guy, successfully divorced 4 years now, (me), has done years of therapy and it helped immensely.It is not a sign of weakness but it is like going to the dentist to get your tooth fixed. It will make you feel better when it is done. Most people only know each other's persona so it is no surprise you say, "you think you know someone.. " He is showing you his real self right now, and it is hurt.

Dear Jo-
I also got sucker-punched.  Our teen daughters knew six months before he told me.  He was detached then angry, then friendly, then detached. He also didn't want me going to a lawyer, also didn't want counselling or mediation.
This changed when I found out about the girls knowing every little detail, so I had him served(he'd moved out to his own apartment).
In counselling he blamed me for it ALL.  That counsellor will no longer work with him, and tells me not to let him bully me.

It doesn't matter if it is a mid-life crisis, a fling or a personality disorder- you have been in this for 20 years and deserve a certain amount of respect.

You are significant in this world, you have rights.  Don't forget your needs and wants in the coming times.

DNM

Your husband's anger may be something worth thinking about. Do you have any idea what he is so angry about? Does he have any idea what he is so angry about? It's tough ground to trod.  His being cold and detached might must be from his now really knowing what his anger is attached to. Take care of yourself now. It's your main responsibility.
best,
Ken