You are in shock. And that is understandable. Change is hard. More so when it isn't what you want.
Gather people around you for support. Seek your own counselling. And contact your gp if you feel you need too.
If he wants to leave there is little you can do. He has every right to end a relationship. I know that sucks right now for you.
You also know that you will be ok eventually. Many others have walked this path.
Like you my future with tied up with the kids dad. The plans and pensions etc. I was devastated. But then drew a line. He had made his choice so I stepped right back.
Contact with the kids was eow. He didn't get to skip off to live his happy life with ow child free
Money through Cms.
I cut him off from me in every way. Physically. Emotionally and practically. I went through all the bills stopped taxing and insuring his car. His mobile phone bill etc.
I claimed the benefits I was entitled too and increased my hours at work
I went through the whole of the house and gathered his belongings up in one place. Moved things around. New bedding flowers etc. Little things to make it feel different.
I got counselling. I talked the ears off of anyone who listens and journaled my feelings. Now looking back what was written was rather lame. I'm glad I didn't send those messages and emails to him.
And I cut him off. He has no right to ask about my welfare. He has made it very clear where I stood. I'd engage about the kids money and the house.
He thought we could be friends but the betrayal I felt was too great. And he has behaved pretty badly since then so I still get very tight boundaries.
4 years down the line. The kids are the happiest they have ever been and doing well. The house is sorted. Money is tight but it is ok. I know what is going out and don't have someone else spending like there is no tomorrow.
His life doesn't seem to be so great. From an outsider prospective. He lost his well paid job and several others over the years. The kids hardly see him. He has a rented room somewhere and ow isn't on the scene any more. I genuinely hope things work out for him. The children and I have a good and happy life and I'm grateful for that opportunity.
You will heal in time. I know you don't want to. But have dignity. Go to pieces with friends and grieve. Tell him to go. And to go now.