Kakelady, I am sorry to hear you had a bad Christmas and year last year. The important thing is that your DS is doing well. Like some of the others have said, the plan is ambitious if you follow it to the letter.
I am one of the FT working folks, my DH works overseas 50% of his time (and works when he's home too) and I have a DD with SNs (HF aspergers and ADHD) - so life is full on in our house. (We have an au pair too since DH started working overseas, which helps but also makes it more complicated!!).
I look on the HGP as an opportunity. It gives me headings to work on when I have 5 minutes. It gives me a room to concentrate a couple of 15 minute blocks on, to clear the clutter and maybe get some deep cleaning done. It makes me think about how I want the holidays to work and how I need to go about that.
It does NOT make my house look like Martha Stewart's house, or like a professional cleaning crew have spent a week shining it all up. It does not guarantee that I am not going to be mailing a few cards on the deadline day (or later) or wrapping the last few things at midnight on Christmas Eve (or as we leave the house on Christmas Day). It does not result in me showering everyone around me with homemade mince pies and cookies, and giving beautifully handcrafted presents.
But there's a chance that the house will be relatively clean. I may get 1 batch of mince meat made, so I can give it in jars or use shopbought pastry to have 1 batch of pies. I usually manage to save a few $ by finding things in sales (and 1 year, I was lucky to buy something special early so it was still in stock!!). I always have something for everyone on the list, and although that is sometimes a very generic "that'll do" type of present (like bath stuff or a bottle of wine), it is usually something that is a good match for the person and their needs/interests (which may BE bath/wine etc, but their favourite type, or something quirky for them - doesn't have to be expensive, more that it needs thought before going to the generic mall stores in a panic to get everything in an hour - internet shopping is GREAT!!, especially in the middle of the night or, in your case, times sitting beside someone while having a treatment so you are there, but can be on a device to research and even shop together for others, or while the patient naps during treatment or you are waiting in waiting rooms). I am far less likely to miss out on family events by ensuring I've booked tickets and organised diaries to not clash with other events. I'll have, maybe not new clothes - but clean, nicely put together outfits for the different things happening. I will have made sure I have put a few minutes aside at critical points to have a bath or take a walk so that I can unwind a little too (and I have the nice bubbles for the bath, or have checked the forecast to have firewood and hot choc ready when I get home!). I'll have my cards list made, so that when I have to take a trip or know DD has an appointment, I will bring the list and a pack of 10 cards with me, I might write none or all 10 - but little spots of time get used up practically that way. And I have some opportunity somewhere to give something back and remember the true spirit of Christmas.
And despite November-February being absolutely manically busy at work, I'll have done enough that Christmas is nice, I am not turned into a gibbering wreck in the corner, the family gets to do a few nice fun things together (and we are not running around stressed to the max doing shopping) in December, and we have enough things in to allow us a nice dinner and to entertain any visitors without pigging out on the 20 boxes of chocolates so I can't fit my clothes by Jan 1st!!
And best of all, by having a plan, and making use of the little spots of time I may get to tick off small things from my list, I suddenly realise one day that I have a LOT of things done and that really helps reduce stress levels too. (That, and learning to lower my expectations a few years back - once everyone is warm, comfortable and well fed, all the rest are trimmings. You don't have to spend $$$, or have the "perfect" house/decorations/feast, to have a good Christmas).
Sorry, I am not meaning to preach, just try to reassure you, remind myself (and possibly trying to avoid looking at real work things now that I am back chained to the desk again!).