Dear OP
I take no pleasure in reading this thread and even less pleasure adding to it. My heart goes out to you.
It is a known phenomenon that most men generally don’t leave one woman unless they have another lined up. Maybe it’s because they don’t want to give up regular sex/ having their clothes washed/ dinner made. Who knows.
Your husband left you for another woman. However, it did not go as planned and the ow ran a mile (back to her husband). She was probably in the affair for the benefits and when it became real, she opted out. Your husband is failing to acknowledge this as it will show him to be a fool.
Your husband is having trouble accepting how this has turned out. He is mooning after her and ‘grieving’. He has separated from you ...just incase. He’s proving to OW that he is all about her. He’s waiting, just incase. Whilst he is away from you he can text her at his leisure, possibly beg her to reconsider. Stalk her social media and wait.
However, he realises that she may not come back to him and because he doesn’t really want to end up alone, he’s keeping you waiting. He’s telling you he’s doing this to save the marriage...it’ll keep you hanging about. Think about it, if the ow changed her mind today, would he still be trying to save his marriage to you?
The anger is due to you not being her. It’s not fair (to him at least). He is angry at you, because who else is there he can be angry at? He can’t be angry with the OW, as he has her on a pedestal. So, he’s angry at you, for no other reason than, you aren’t her.
This is an appalling way to treat another person. It is beyond selfish. It is cruel.
The therapy is fuelling the issue, because it’s allowing your husband to feel justified in his actions. You should be allowing him to grieve. You should tolerate his anger. You should give him this space. I question the authenticity of your therapist as I’m pretty sure that this is not how it’s meant to be.
The obvious answer is also the hardest. He doesn’t deserve you. He deserves to be alone and miserable. You should tell him to keep the flat; that he won’t be coming back and move on without this pig of a man. But after 40 years, that’s easier said than done.
As crushing as it is, stop for a moment and know that even if you get back together, you are his second choice and not the person he wanted. You will never live up to what she was - after all he never got to see the real person, only the fantasy of what could have been. You can’t change what’s happened or him.