He has been planning this for a long time and you are catching up.
Be kind to yourself. Eat and drink when you can. Go to your gp for support and consider medication. It will help settle your over whelming emotions. Tell people. Gather anyone around you who will listen. I never thought I had friends. Just people I say hi too. When ex left I went and cried in my neighbour. They were happy to be supportive.
Have a look into counselling. Some charities do low cost service. To give you a safe place to talk things through.
Start a journal. Write it all in there. I wrote draft e mails. With all my emotions but never sent them. In reflection they were embarrassing.
Start moving his stuff. He wants out so help him. Gather his belongings in one place. Spare room. Attic etc. Have a good sort through the whole house. Move the furniture around and buy new bedding. Small differences help. My ex took great offence to this. That was because I was taking some control back from him.
Gather any paper work and copy everything. Bank statements etc. Get a good solicitor. If there are hidden assists. And pensions you have a fight a head of you.
Look into benefits council tax reduction etc. Tell work and schools. Let others support you. This close to Christmas sucks so think carefully about how that looks. And do the right thing by you. If him being around isn't going to work for you then don't have him in the house. Arrange contact for the children. He doesn't get to Swan in and out of the house as if he still lives there. Time to our firm boundaries in place.
I know how broken you feel. It has taken me the best part of 2 years to get my shit together. But you know you will survive because you have too. Keep posting here it really helps to have a sounding board.
And be prepared for a. Ow. Yes some people do leave relationships just because they have broken down but I am sure you have read enough mn to know it is usually an ow.
Go as low contact as you can with him. Fake it if you have too. Money through cms. House etc through solicitor.