I am so sorry you are in this situation. I wouldn't wish it on anyone.
Don't let him take advantage of your love for him now. He has checked out and prepared for this long ago. You have some catching up to do. Practical thing are easier. And will distract you. Emotionally that will take longer. How long depends on your actions now.
He is not your friend. There are enough post on mn you know this. They tell lies and twist the truth for their own gain. This is now a business arrangement. Anything agreed in writing. Text or emails. Keep a trace of things said.
Gather paper work. Pensions. House. Wage slips etc. Keep passports somewhere safe away from
The house.
Tell him he has to go, That what he is doing is emotionally cruel. And if he had any respect for you he would of gone by now.
Claim any benefits you might be entitled too. Turn2us website has a calculator. CAB will only sign post you there.
CMS also has calculator. Ask him to set up direct debit. He has one chance at this or you go for deductions of earning and formalise this through CMS
Go through bills cancel anything that is his. Car insurance mobile phone bill ( I had my ex storm in 3 months after he left asking why I hadn't paid his phone bill. Whilst he had paid no child support!). Stream line your out goings so you know your expenditure.
Legal advice. Some solicitors will offer an initial consultation for free. Knowledge is power. So do not let him rail road you into any decision.
Contact do not be obstructive about this. It won't look good on you further down the road. And you won't get him supervised access only ( there are violent parents out there who do not have to go through this). You can ask him to not involve OW. And I hope he respect this. Again there was nothing you legally can do about who or how he spends his time with DC. So long as they are safe. This works both ways. He is not obliged to know anything that occurs in your time in your house.
Emotionally. You need to grieve. Be kind to yourself. Gather good people around you. Get support of your GP or counselling if you need it. It is a safe place to vent and work things through. Do not off load on him. Ask him why etc. He will only tell you what he thinks he needs too. Keep a journal. Do not out pour to him. Any communication is text. Or email. Keep it business like. And formal. Take emotions out of all communication. Sit on any thing for an hour up to 24 hours. You do not have to respond immediately. I took great delight in taking my time in reply's for anything none urgent. My ex made me his last priority so I wasn't going to make him high up on my list either.
Right now just do what you have too to get by. The children and I were treated really badly by my ex. It took me a long time to be ok. I worked. Kids got to school fed and clean. And for awhile that was me winning.
Let those who love you support you. They want to help.
You deserve so much more than this. And you know it. Stay strong.