Kate, the folks here will provide you with much wisdom. I am in the middle of a situation very similar, as most here have been themselves..

You are both torn apart. The difference is that you possess logic and reason. He does not. He is filling his thoughts with emotion, fantasy and only for the very short term future. That is his portal, whether it is healthy or not. He will do things you never thought possible, clothes, working out, spending, etc.... some go on to do drugs or abandon their families in order to live out what was missed.

The best you can do is 'do for yourself.' He is now selfish, narcissistic and is doing everything for HIM and HIM only.

If you seek reconciliation, I suggest one of the coaches on this site. I had success with them. If nothing else, you will learn important life lessons, learn about yourself and some human management skills. Stuff you will carry forward in your personal and professional life. I had an early reconciliation (2 months after BD). It lasted about 6 weeks and ended in catastrophic failure.

This takes time. I don't know how long but plan for a long haul (years).

Do not engage in emotional talks. He will try to troll you into battles to justify why he is leaving. Deflect, ignore and use phrases like "that must be difficult for you" or "I'm sorry you feel that way."

Do not get caught up in what he says. Believe nothing. Watch his actions and buy into half of those.

Do for yourself. Be a person only a fool would leave. In the end, you will grow and be ready to move on, in one way or another.

Pour yourself in to your children. Get help from family and friends. They don't need details and are gold mines of support. He will likely surround himself with yes men who support (or are ignorant of) his fantasy. My W has abandoned her entire family, so that may happen as well.

It is a bumpy, emotional ride. Feelings come and go. With time, they will space out and you will be more level. Please continue to reach out to the folks here. They are invaluable and deeply care. You can PM me as well if you need anything.