cat, I haven't read what others have said to this, and I bet they gave great advice, but I've got to chime in here.
I was like your H when I was younger. I kept a relationship going because I felt responsible. I took the girl to a C to get her help. The C called me in the room and told me "you aren't Jesus Christ, you can't save everybody". I needed to hear that. I repeat that to myself when I find myself getting involved with psychotic people and trying to help them. You're H needs to hear it too.
But, and I think this is really important, it's HIS crisis, his decision,his pain. He needs to figure out that he has to fix this or go down with the ship. If he stays with the OW, his life will be hell, but it'll be his choice and he'll deserve it. Even if you want to save him from that, you can't. You're not Jesus Christ either. He has to figure it out and find the strength, for himself.
You have to cut loose (not give up on the M, but detach). Do what you need to do for you. Separate, stop talking to him, separate finances, divide the house, whatever. No maliciousness, no hatered, just taking care of yourself, and the kids. He isn't. You have to. Divorce? If he wants to. It won't destroy you, and you'll be out of the hellish mess he's made of his life. If he gets his act together, finds the strength to do what he knows he has to do, or should know, and you want to be with him, great. But cut loose now. Cut him out. don't be co-dependant. Dont' be an enabler.
You do what is right. Stop helping him. He has to fix this. You can't. Take care of yourself. Be an examle for your kids. Your H isn't a bad man. He is hurting. He wants to please and save and help everyone. He can't. He has to figure this out himself. He has to own his mistakes and learn to move on, find the strength to help the OW the only way he can, by cutting off all contact with her.
For him, he's being blackmailed. There is only one way to deal with blackmail. Come clean and face the consequences. It could be terrible. He could lose his job. Many people have lost their jobs. There are other jobs. Either way, he can't keep going with the OW holding this over his head. And YOU cant' be party to this, helping him drown slowly instead of swimming to shore. You have to detach. Don't enable him to continue, by your actions, his destructive behavior.
I've been in his shoes, in a way. The girl in question claimed to be pregnant. She threatened, and did, tell my friends stories about us. I tried to help her, she tried to manipulate me and use me. I found the strength to cut off all contact with her. It was hard. I felt like an awful person, but it was best for me and for her. Your H has to do this, but can only do it on his own.
Cat, be strong. Just like your H has to find the strength to fix his life, you have to find the strength to detach from him. It's the only way to help you and help yourself and your kids.
cat, I'm praying for you. Find the strength. I'm not saying Divorce. I am saying do what you have to, for both of you, for all of you.
M45, W45,S15, D10, Bomb 10/3/06, Moved back in 11/6/06, finally ILY 9/07 last thread