(((( HUG))))
You'll get through tomorrow 5 minutes at a time...you will, because you are strong.
I know it seems like he'll be great about everything, but a word of warning. USE this time to get the very best deal you can, because the guilt soon passes & the reality settles in. I'll eat my iPad if there isn't another woman (though it may take time for her to come out of the woodwork, whether it's the one you suspected or not). I know you say that's by-the-by now, but it's not if she's in his ear about what he should do/say/settle on.
Please go and see a couple of solicitors (at least) and find one who has a bit of fire about them. Known around here as a SHL (shit hot lawyer).
I'm sure the kids will be upset, especially your DS - and no, they don't deserve this, but hopefully your STBExH will reassure them that he loves them & spend as much time as he can with them (NOT in the family home - that confuses them, will upset you & gives him a false sense of still having a right to be there & treat it like home. He's lost that right now & needs to accept it).
Try not to take it to heart when he says he hasn't loved you 'like that' for ages, he was lonely, he was blah blah blah. It's justification in his own head for doing this. He could have talked to you (not just one brief conversation about doing more together) & he could have told you how he was feeling - but he didn't. He's chosen someone else because he's a selfish git who has had his head turned by someone giving him their undivided attention, with all the lust/newness/excitement of 'someone new'. He's been a fool, one day he'll realise that - but it's HIS doing, not yours.
I know how hard it is when you still love them, still want them, still trust them - but please try to understand that we've all been there, many of us still loving them & thinking 'he's not like those other men, he'd never not look after me/the kids' etc. It's a false sense of security in the situation & the longer you linger in it, the more damage emotionally & financially there will be.
Its so very, very hard when they're your life & you have your whole future planned with them - for me it felt like he'd taken my whole life, my future. But (eventually) some wise person said to me 'He hasn't, the rest of your life is ahead of you, a blank slate - get out there & make NEW plans!' (I think it was that or hit me over the head with a bit of 2x4 by that stage!!).
You're not alone x