Good post Maleview, my thoughts also after reading the whole thread. I too found that we get on better, years down the line because I feel that we've both grown up to some extent, matured, regardless of our age. It's nice to read that you're happy now too. 
dusty
Thing is, as Chipping said to you, you can't change your past, or his. But you CAN control the future.
Okay, I have now read all of your posts, and a few others!
He got caught out and admitted it to his Area Manager. OW was moved to another store (as punishment?) but this all started when he used to drop her off after work to save her walking home in the dark?
I'm so sorry (and angry) that your DD found out about his affair and had to tell you. Poor girl. No wonder she was angry with him, putting her in that position deliberately or not makes him a selfish idiot.
I agree that you need some real space as others have said but dusty - promise yourself that you won't feel sorry for him at anytime during this process, or guilty for what you did to his partner before you. Some might say what goes around, comes around (not my thoughts here btw) but I think you already knew what you were getting into. You fell in love with him and him you. It happens. It's what you do about it I guess........
But I do know one thing, he is not worth your sympathy. You are just as important as anyone else. You need to have time out from DS and from the day to day shit that you are having to deal with, all the ins and outs, paperwork, informing people of the split etc.....
Do school know? And DS's docs/Paeds/SENCO etc? You could really do with some help with explaining things as they transpire to him, IIRC, you said his current SENCO wasn't so great, lazy?
Does he, DS, have a respite worker who can come in and give you some space? Or anyone else?
I really hope that you take on board GreenMunkies's (SP?) post, and Mollymoo's too.
I'm absolutely certain that my DH has felt like bolting some days, life is incredibly hard with a child who has SN, in fact, the lack of sleep alone is enough to drive you to drink! ADs can't help with that, I know. There are days when you'd sell your soul to Satan for an hour of unbroken sleep.
DD sounds alot like my DD, she's 12. DS (aka Nemo) has Complex Special Needs and 24/7 care. He's 2.5 so there is a difference in age but still, I always feel guilty that he has the best of me IYSWIM? And she has what's left, as does DH. Children with additional needs are full time, full on work.
Your H has to help. He knows him too, which means he can help. Familiarity is better than strangers. As is routine. It must be so hard for your DS.
Your world's have been totally turned upside down. If you look back though dusty - it's been a month to the day since your first post.
Look how far you've come! Look at you now. Yes, you're sad, and hurting, angry and grieving. Yes you could slap him six ways from Sunday but look at YOU.
What happens from here on in is all down to YOU and do you know what? The DCs will be just fine.
Personally, I think taking him back would be a mistake. Certainly doing that any time soon. You need to heal and to rebuild yourself. He needs to prove that he does want to be with you for the long term.
And your children need to learn to live with their parents apart for now.
No easy task I grant you. You and the DC need to learn to live without him. Again, not nice but it is necessary for a while.......
Don't worry about Christmas, Birthdays etc...... take it a day at a time, an hour, whatever works for YOU. Agree to nothing until you have time to think.
So if he asks you something, tell him you'll get back to him.
Talk to your friends and family. Let them help you, let them in. And be kind to yourself. Stop with the what ifs and maybes. You can't change what has happened.
Find the real YOU again, what YOU want. For you and the children xx
Sorry for the epic post! 