You said your husband told you the OW would never leave her husband? Pretty sad if that's what's stopping him. So if she would leave her husband? What then?
As for your husband not wanting to discuss this with you. An A (EA + PA) is like an assault on the BS. Both an emotional assault as well as physical.
If you called out to your husband that you were stabbed (and blood was gushing out everywhere) would your H say, "I don't want to talk about this"?
Even worse, HE'S the one who STABBED you, and he doesn't want to talk about it? FUCK HIM!!!
Listen, we certainly understand how painful this all is for you (and rightfully so). Yes your M was in a bad place before all this, but if there's ANY chance at all of saving it you have to put your fear aside and do what's right. Without trust in a relationship there's no foundation and no stability and everything WILL come crumbling down eventually.
You've been given great advice.
Cast light on this immoral act.
Tell the OBS as he deserves to know.
If the roles were reversed and he knew wouldn't you want him to tell you?
Tell your family and his family.
Give your H a date that he has to have a new job or you D.
Make him take a poly as you obviously do NOT have the truth.
Speak to an attorney so you know your legal options.
Stop doing everything for him.
Don't cook for him, wash his clothes, run errands for him, make him sleep in another room, just do the 180 on him until he realizes you are NOT F'ing around any more with this and if he truly values you and the M then he'll do whatever he has to do to show you he's being honest, making you feel he's a safe partner and validating your feelings with all of this.
If he's not willing to do this then why would you want to stay with him? Yes there's kids involved, but think about how you feel right now. Can you imagine this being your life tomorrow, next week, next month, three months from now, a year from now, two yrs from now???
That's no way to go through life.
You have ONE opportunity at this thing called life.
You are dealing with all of this from a position of fear. My advice would be to get PISSED off and say NO MORE!!! This man is supposed to be your H, someone who took vows to treasure you and to protect you and FORESAKING ALL OTHERS.
Is he doing any of this?
Why are you putting up with this crap?
Go tell his parents and family.
Go tell your family.
You are strong enough to deal with this.
Do what you KNOW you have to do.
Tell the OBS first and as you've already been told do NOT tell your H you're going to do this. Just do it!!
Then tell the OW you know as well (again without telling him you're going to do this).
Time to TAKE ACTION DESPITE BEING AFRAID!!!