It helps if you think of him as an addict. Or even a pod person. Ever see the movie Men in Black? Where the alien ship lands and the bug alien wears the guys skin? The wife tells the MiB it wasn't Edgar. It was like something was wearing her husband as a suit. That's how WSs are. They are NOT the people we fell in love with and M.
It helps to think of them as someone else when you're dealing with this. Mourn him like he died, because the man you loved isn't there right now and may never come back.
The 180 is here:
Don't text him. Don't contact him except kids and finances. Don't look for reasons to contact him. Right now, your contact is a bonding point for him and the OW. Take yourself out of the equation so they can fully experience each other as the lying cheaters they are.
Whatever alien that is inside of your WH needs to feed off of validation. This is the addiction. One person wasn't enough to fill the emptiness inside so he found someone else. The thing is, he needs two people to feed this hunger. Take yourself out of the equation. You are worth more than being used by him to feed off of emotionally.