Hi OP so sorry you are going through this.
My partner left after 22 years. I won't go into it as it's not quite the same. But I did learn an awful lot about the pain and complexities of long term relationships ending. And the lying. Not just the immediate lying about the affair (in my case) but the lying that had been going on for years about how he felt about the relationship. During one heated argument he screamed at me that he had been 'managing' me for years. He later took that back saying he had wanted to hurt me - but actually I think it was true. He knew he wanted out at some point, but parked those feelings because it was inconvenient, difficult to leave and I think he wanted own son to be grown.
"Friend", "nothing to do with him leaving" ? won't tell you who she is? I think it is outrageous he won't tell you. Without a doubt he has been sleeping with her. I think you have likely been 'managed' too.
He's managing you still. But make no mistake he has left. Friendship? My ex wanted that too, in fact I think my ex wanted me to help him recover from the relationship he had broken. If I had tried to be friends I would have been eternally caught in a cycle of buried longing. And he wouldn't have cared less.
It did take me about 7 - 8 months to get to the point where I was strong enough to cut all contact. I have not spoken to him in 3.5 years. It was my choice to make it so extreme. It was the only way to survive and regain self-respect.
Personally I would not stand for the witholding of information about the other woman. He's witholding for a reason. She's known to you, or married and leaving her partner. It may have been planned for years. If you can afford it (or make him pay) get a good counsellor (forget relate).
Don't feel bad you can't break contact yet, but work towards it. But you do need to see his bullshit for what it is. You are been manipulated to play a pre-designed role in a breakup he has planned and he is controlling.
good luck and courage my freind.