I hear you loud and clear OP, and everyone else who is feeling overwhelmed.
I have adhd and this is pretty much how I feel during December every year unfortunately.
My advice would be to consider the following:
Set a timer for 5 minutes and do a 'brain dump' onto paper of everything in your brain pertaining to Christmas and the next few weeks. Just get it all out and onto paper.
Book an online grocery delivery. Do this asap, but just put anything into the basket and then checkout so you have got that delivery booked. Make a note of when the cutoff is to edit the delivery, and write this down / put it on calendar.
Focus on what really matters. Try to really nail this down, because there's probably a lot of stuff that you'd 'like' to do that just isn't important. Let the unimportant stuff go.
It sounds like what might matter most to you is that DC have a lovely day. If so, make a list of anything that is absolutely essential to making this happen, and a second list of other things that would help but are not essential.
Be brutal in making these two lists. Do DC need the house to be tidy? Do DC enjoy a huge slap up traditional Christmas Dinner?
Do you really need to send cards?
Get yourself on Amazon and order what you need to buy for gifts. Bear in mind that 'the perfect' gift does not exist. Post on the Christmas board on here if you need ideas.
Also if you have the budget buy a few extras that will make the run up to Christmas day memorable / fun like a board game, or jigsaw, or baking kit .
Bear in mind that it's not only visual decorations and gifts that make us feel Christmassy- so introduce other things to usher in the Christmas spirit:
The sounds of Christmas: play music, sing carols, pull crackers.
The taste of Christmas: grab easy Christmas themed foods- chocolate Orange, mince pies, candy canes, Yule log, quality street.....
The smell of Christmas: heat the mince pies, light candles.
The touch of Christmas- blankets, crackers, cold walks, jigsaws.
Hope some of this helps a little!