I know all about this. With six years of hindsight, I would say firstly, and most importantly, this is ALL about him, and not remotely about you. DO NOT TAKE IT PERSONALLY or it will eat away at you and stop anything positive developing from this.
IF you want him back you have to stick on your dancing shoes, and make like you don't care. Do not let him see that you are heartbroken, just sort of shrug, and do your own thing. He can only think of himself and what he wants and will be surprised and confused if you are the strong one who takes this in her stride. Be careful, as I am sorry to say, he is no longer your friend.
He is experiencing severe change and has absolutely no idea what he wants which, as you have found, changes from one moment to another. You can't fix this, or make it better, you can only look after yourself and your children if you have them.
His brain is a foggy confused mess, and he is like a cardboard cut out of the man you knew. He can only see things from his own perspective and you need to let him realise what he is missing in losing you. But he can't and also doesn't want to listen to you and to justify this craziness will invent and twist everything around to make it seem like its all your fault so don't indulge him in conversations as this can be very painful.
Definitely don't involve yourself in the pick me dance, and sadly, don't expect the man you loved to ever reappear. If you do end up back together, the MLC can last for about 10 years and he will never be the same man as he was. Think only of yourself, what you want, need etc as I'm afraid he just isn't capable of caring for anyone but himself right now. The secretary is getting a very short changed version of him. you got his best years and no one can ever take that away from you.
There are some great websites, One is GuyStuff and another by a woman called Pat Something.. I would look them up and read and read and read some more to help yourself make sense of it. I'm so sorry, it was the most painful experience of my life, PM me if you need to. I know that right now you want nothing more than what you thought you had for the last 20 years, but hold onto your hat, and the chances are when you get through it, you may be surprised to find that you no longer want him. In the end he will be the one who is grieving for you and you will have moved on to better things.