He has acted according to the script then, wanting to backtrack from telling the DCs about the OW. Don't let him. Keep reminding him as the date draws near (Friday isn't it?) that you will be revealing all about the OW, even if he doesn't.
I'm not surprised he's changed jobs rather than face his colleagues' contempt. I think I said in the early days of this thread that I thought he was a man who had a very high opinion of himself and placed disproportionate value on the esteem he was held in by others. I expect he will lie when he goes to his new company and claim there was a gap between relationships; after all, that's what the OW did, wasn't it?
Solost I'm also not surprised you are feeling low atm. Strangely enough, whenever I think back to the Christmas before everything happened to me, I have always shuddered, because little did I know it at the time, but so much pain was ahead.
So rather than having more positive memories of last year's festivities, I think it would be better for you to reframe this. I know I keep telling you that in truth, your H hasn't changed and I expect you feel defensive when I keep saying it. But last Christmas, your H wasn't a wonderful selfless man you know. He was a man who would very soon give himself permission to deceive his family. Ignorance about him was never blissful, really.
This time of year is really hard, because of events at the school and all the childish innocence and excitement from the DCs. It reminds us all of a time when life seemed innocent for us, too. It would be almost impossible not to feel maudlin atm after all that has happened to you. I hope you understand that and I sympathise hugely.
I know you will also be fearing telling the DCs and I know it was a conversation you hoped you could avoid. It is no wonder you're feeling low and defeated. I'm pretty certain your older one won't be completely surprised though. IME, it is the older ones who need more attention and explanations and yet also want to retreat into their shell and not ask too many questions.
WRT Christmas itself, be guided I think by what the DCs want to do, after they have heard the truth. I would also suggest if you can, to create a few new Christmas traditions and memories, so that you will be less tempted to recall happier times.
Thinking of you and sending you strength.