Hi Trudeau. My H of 28 years left at about this time of year 4 years ago now. For 6 months I'd had, we don't want the same things, we have grown apart, I don't know what I want and I need time to think about that........ I thought it was a bad patch and we both cared enough about each other to work it out.
The children were 21 and 23 at the time. The youngest and her boyfriend were living with us (she'd got pregnant, bf was abusive, they moved in with us and at the time of my H leaving, the baby was about a year old) and that had been very hard for me, my H coped by going to work knowing I was handling things.
However.....there was an OW, and he'd made his mind up to go, and once that was clear, he did leave. I empathise with your feeling that your H is way ahead of you and you are playing catch up with your emotions and your responses. Mine made his decision and got used to it long before I had an inkling of what was really going on.
Anyway, I was present, but I made sure my h told the children. It's his decision, there is no reason for you to make it easy for him.
2 other things: my H refused to see the DC on his own (without OW) and said he was leaving because he deserved some happiness now. The DC felt really rejected and both felt that the past had somehow not been what they thought it was and felt destabilised (especially the older DD expressed this, maybe because she had no significant other to offload onto).
Second, I think it's hard for older children, and they are your children, it's not right to see them as adults who can cope. Mine both acted out to me, it was as if they couldn't rely on H, so they didn't try to, but they tested me over and over again. Be prepared!!
And 4 years on, I'm glad he went, and I'm glad it was his decision not mine. I remember, especially at this time of year, how utterly bereft and hurt and sad I was, but please trust that it gets easier, one learns to accommodate it, with time. Take care of you, and if you can, I'd recommend counselling for support for you. You are really important and the next few months will be tough. And get a decent lawyer.