It is a shock for you. He started checking out a long time ago. And there may or may not be another women. That isn't for us to guess. But you know from reading mn it is usually the case.
He is not the person you know and love. And he has rewritten history and will find fault with his life with you to give him a reason to do such a shitty thing.
Life goes on although you feel rooted to the spot. I remember ex telling me we would all be fine. And I told him of course we will there isn't another option is there.
I know how you are feeling. I wanted to beg and plead. We weren't in a good place but I want to work on us and put it right. My ex had ow lined up And moved in with her 12 weeks later.
Time to get tough. As hard as it is. Show him no emotion. Contact is not at the house. He comes and takes the children at agreed times. Children need routine.
Go to cms site and work out money. Claim benefits. Single person discount on your council tax. Get all bills in your name. And stop any direct debits that are his. Phone bills car insurance. Etc. He wants to be single he gets no benefit in you organising his life
Gather people around you. They will want to help. He is no longer your friend. Keep contact about kids and money only. Anything else is no longer his concern. Show him you can strong and independent. Even if you don't feel it.
Copy wage slips. Pension stuff etc. Anything you think might be important.
Bag up his stuff. Start moving things around to change Home a little bit. But new sheets for your bed.
He may see what the future looks like and change his mind. He may not. Whatever happens you deserve better. I wanted someone in my life who would stand by me and the children even in the most difficult times. And that wasn't my ex.
I'm 8 months down the line. I still cry some days. I miss the life I had and the future I thought I had. But it is less raw.
Just take each day as it comes. Do what you can to get by. Eat and seek professional support if you feel you need to. There is no shame in it. You are dealing with a bereavement.
How you feel today is not permanent. This will pass. But right now you need to recover.