Painsme,
I'm so sorry for what you are going through.
I don't want to be the one to give you this advise but here goes....
If he doesn't want to talk to you about this or even try couples counseling he doesn't seem to want to work on the relationship.
It's pretty clear to be me he was having a "relationship" with this other person. Most likely physical...betrayal is betrayal. Lack of respect is lack of respect. Someone who is not WILLING to even make an effort won't.
Maybe it's time to think about your own needs.?? You are worth it. Don't ever let anyone tell you otherwise. You can't change him but you can work on changing yourself. At the end of the day thats only thing anyone really has control over.
No, it's not your fault. Don't believe this for a minute.