I'm so sorry you're going through this. Your head must be all over the place.
I got a very strong whiff of OW from your first post. I also thought I was married to a 'good man' who would never cheat on me, he worked away all the time and came home exhausted every weekend. Not surprised he was exhausted because it turned out there were actually 2 OW, one of whom he'd even had the nerve to introduce me and our DC to as a work colleague, the OW didn't know about each other.
I'm really sorry to say this but at this point divorce is likely to be unavoidable. He's checked out already. Please don't hide your head in the sand, hard as it might be to let go when it’s instinctive to try and work things out, because you don't want to give him the upper hand if you can possibly avoid it. You are doing this for your kids, their father has let you all down, please salvage what you can for them.
So practicalities, my ten pence worth, and there's a load of really good advice on here already:
Get a solicitor appointment asap.
Move as much joint money as possible into an account in your name only, your solicitor will advise you not to spend it, (don't) but it will stop him spending it if he can't access it. He might try to dispose of or hide assets.
While he's away this week, gather original paperwork - birth certificates, passports, marriage certificate, bank statements, any investments etc. for all of you. Make copies and put them in a safe place, keep the originals where you can easily access them at short notice. Make a note of his NI number too, might come in useful to know it later.
Gather together all of his pension details if you can - the provider/s, contact details and policy numbers, as you don't want to give him chance to hide any (this happened to a friend, her ex claimed he hadn't got a pension, he worked in his family business so she couldn't prove it).
It doesn't sound as though he has any grounds on which to divorce you, but from what you've said you can use unreasonable behaviour to divorce him. Get copies of bank statements with the porn subscription payments on. Your solicitor will advise you on this.
Try to take control of the situation, decide what outcome you really want for your DC & yourself wrt access, bear in mind the greater % of time they're in your care, the more he'll have to pay you in CM, but the less 'freedom' you'll have. Also hold off on getting a job for now, you'll get a settlement that's more in your favour.
Never hold back on taking him for as much as you can, he's chosen this, not you, the selfish pr* isn't giving you or the DC a second thought.
Please confide in a RL friend, family member or neighbour, and see your GP if you think they can help too, you are going to need support on several levels while you get through this - but you will get through it, and you'll come out stronger than ever. You are worth it.
Good luck x