Ooh, I hate that. He gets caught doing something he should never have done and now he’s spinning it so he’s the victim and he needs help? Now he’s willing to go to counseling? Too little, too late.
I’m sorry you’re going through this. My best advice for you is to get a good lawyer who can advise you of your rights and how to best protect yourself, your assets and provide for your children. You don’t need to be unfair but you do need to protect your interests.
Please make sure that the children know that it’s not about them. I’d also like to advise that they’re way too young to know about daddy’s little fling. Even though you might feel the urge to make sure they know it’s his fault, resist that. They really don’t need to be in the middle of this at all. If they’re loved by both of you and given information that is age appropriate, they’ll do fine. If they’re school aged the guidance office at school might have a divorce group for kids. My daughter attended one when her father and I split and it helped her quite a bit.
Oh and I second the recommendation for a check up. Have them check for all the known STDs. You don’t know what he’s brought home. If he’s sleeping with you and sleeping with her then you’ve been, in essence, sleeping with all of her other partners.
One more thing (who knew I had this much advice) Start keeping a journal of events and conversations. It may come in handy later if things get messy. Right know you’re in a very emotional situation and it’ll be hard to remember later who said what or what was done when.
Good luck.