I'm sorry you are feeling so awful. It absolutely does hurt, you're right to feel that way.
Have you decided if you are standing for your marriage? That is the first and most important thing to do as far as DB is concerned.
If you have decided to stand, it is important that you leave him alone. He is going to be making some very bad decisions, ones that will not make any sense to you.
Also - the reason people tell you to leave him alone and not snoop what he is doing is because of what just happened - you saw her SM post and it sent you reeling, right? Your job is to get yourself off that rollercoaster.
The WAS is acting on a flush of hormones and emotions, and they are extremely irrational. My advice is to stop looking at what he is doing on social media, stop asking him any kind of questions unless it is related to the well-being of the kids, and do not have any relationship talks.
You cannot control what he is going to do.
You can, however, control what YOU do.
Have you read DB or DR yet? They are both invaluable resources.
Focus on your kids and yourself. Be the best parent you can be.