ITA with the hospice recommendations - mind you, ours is brilliant, has a fantastic counselling team for the whole family; I hope yours is the same. DD1 is "randomly" life limited iyswim, no idea how long she will live so it's easy for me to stick my head in the sand. There was a time when she was a little bit younger than your DS when things were going very badly for several months and though I don't actually know how you are feeling, at the time I thought I did. 
I think the most useful thing is to talk to other people in the same boat: I've always been fairly suspicious of the whole cup-of-tea-and-sympathy brigade but a big lesson in the last few years is how far illness/disability isolates you: not so much because your friends don't want to support you as because they don't have the experience you can't talk to them about it. As soon as you open your mouth they go all bug-eyed and start with the "oh I don't know how you cope" routine which kills the conversation stone dead. You need someone who can say, matter-of-fact: "oh yeah, we had that and..." and then at least you can get to the end of the conversation you were trying to have. You are in extreme circumstances and in extreme circumstances, you think and feel all sorts of things; you certainly don't react in the way Hollywood would suggest, and it's a huge relief to talk to others who have reacted the same non-Hollywood way and find out that you're not going mad, or a bad person, you're just dealing with it.
Oh, and take a lot of pictures. When DD1 got her first diagnosis (she has a few, little ratbag 
) I went through what in retrospect was a period of depression. I couldn't bear to take any photos of her at the time and I really regret it now; I have a dreadful memory and that's a chunk of her life "missing" to me.
Re: Xmas - you can only do your best. My Bad Mother's Guide to surviving more or less everything is if you can't cope or give them enough attention, or be Happy Smiley Mum a minute longer, put a DVD on and feel no guilt, it's not as if you're doing it every day. I have a specific box of Xmas ones that only come out in December to help me survive this time of year. I'm speaking at my father's funeral today, with DD2 in tow. It will be back home and break out the DVDs the minute we get through the door.
Hold tight. The best thing you can do for any of your children - in any and all circumstances - is love them. Everything else is just details.