I'm so sorry. I know how much it hurts and the confusion that surrounds their ability to ignore or disregard our pain and just do whatever it is they want, as if we just never existed. It's cruel and abusive.
You are a real and caring human with a core of authentic feelings. For that reason, unfortunately, you can't just turn it off and stop caring. That's dysfunctional and that's what they do.
The only way to get past it is to go through it.
I don't know the details of your situation, but if he's still living there with you or you're still having lots of contact with him, that's the first thing to stop. Whatever you want to call it, 180, NC except for kids/finances, etc., start it immediately. Turn away from him to focus on what needs to be done, the changes that need to be made, and helping your kids get through the transition.
Now is the time to protect yourself. See a lawyer if you haven't already done so to protect your financial interests. Implement the 180 to protect your heart from more hurt.
It will get better, but it's a process. I promise, one day, you will have gone through it and you will see him for what he is - a selfish, disordered, empty, black soul. You and your kids deserve better. The 19 year old OW? She's about to learn a big lesson about predatory men.