I can't believe that I just fell upon your post. I have this situation, however I am the W. My H is living with his GF. I am home with our kids. Even though they are maybe a little older G(14) B(18) they were devastated when their F left for another woman.
The kids really don't want anything to do with him. They have lost a lot of respect and he is feeling that deeply.
I have told him that I don't want a D. I am committed to working on our M. When we've talked recently he has said he is trying to figure things out. The GF is also married.
I have been praying for the affair to end.
I think the only way he can truly figure things out is if she leaves the scene. But I have no control over that.
So.... until then, I concentrate on my kids and work on getting through each day. When I see him I try to stay as positive as I can. I don't mention the GF or their relationship- I just pretend she doesn't exist.
If you have any other suggestions please let me know..