Hi Lost, so sorry you have been so hurt.
First of all what does she have that you don't? Nothing. You are you, she is whomever she is. Maybe she is a user as well, probably, she now has a cocaine addict living in her house. It's not personal though, it's not about you or your kids, it's about addiction.
If you aren't on board with the addiction, then you can quickly enter the enemy camp. It's like you already had a 3rd person or entity in your relationship (because you did).
His loyalty is to cocaine.
All you wanted was a stable husband and father, is that too much to ask? Not at all. I'm sure before he became a user he may have been that and even after, maybe in small moments. But the addiction has him now.
It's more important than you, or the kids or his family, friends, work, it's more important than him - to him.
I would recommend a couple of things. First of all, take care of yourself, take that energy you are focusing on him and try to turn that back on yourself and kids. It's wasted on him. Eat well, sleep when you can, go for walks (even if you don't feel like it), talk to people, post here.
Read Codependent no more, by Melody Beattie. You will find it helpful I think.
I hope you realize you aren't broken, not really. You have had a devastating thing happen, but you will heal from it. It may be slow, but each day you will feel a little better, even if you don't notice it at first. Steel yourself.
A trip to the doctor is a good idea, also, maybe next week, you might want to speak to a lawyer. I'm not sure what assets you have but you will want to speak to them about custody, child support and finances.
What about banking? Do you share finances? If so you may want to lock down that money (today) so that only you have access to it, for now. If that means calling and cancelling all debit cards for now, then that is what you might want to do. Perhaps go grab some cash for yourself first at the ATM, then call. Cocaine is expensive and he is obviously on a bender. You can then go in to the bank or call them on Monday and sort it out.