It sounds like you need to think of things to keep you busy for a few days, not just the rest of Christmas Day.
Yes, plan a nice breakfast with DD and opening her presents with her. Have a good Christmas playlist going.
Once she has gone to ExH, plan your day to include cleaning up after she's gone, and getting yourself comfy and relaxed.
You will need at least 1 more meal - you don't necessarily want a lot of fuss, but some prepping and cooking may help to keep you busy - so plan something that you love, don't make yourself very often, but that is not too complicated to put together.
And have other easy to prepare and eat things to snack on as well - that you enjoy and are a treat to you. And while alcohol is out, you should still get yourself a drink that YOU really enjoy and don't have often - if hot chocolate, a really good one with proper cream for putting on top; posh bottle of lemonade; seedlip and tonic (I think it's seedlip is the alcohol free "gin"); good ginger beer (alcohol free type); etc. Or even just your favourite squash but made with sparkling water and adding a slice of fruit or mint leaf as garnish.
Something to pick up and put down over the few days would be good. Like a jigsaw, colour by numbers, good book or new magazine with lots of interesting articles (New Yorker, Spectator, Phoenix, National Geographic) - I know they are not the cheapest magazines, but the same kind of price as a book and usually very interesting but short pieces (especially if a book is too much to keep your interest).
Make your house comfy. Light the fire if you have one. Have candles or lamps rather than full ceiling lights. Bring out any nice blankets you have to curl up under when reading/watching tv.
Get out into the fresh air when you can. A walk every day is good to have generally, but if there are any scenic spots within reach, take the chance to get out and go for a good tramp around. (Have the water organised to have a long hot shower or bath when you come in - so getting cold and wet doesn't matter because you will be warming up afterwards). (And have the makings of a warm drink ready too before you go out in case you need it when you get in - kettle filled and mug, teabag/coffee/hot choc ready beside it, and maybe the pack of biscuits or mince pie to enjoy with it). Yoga on youtube could be something to do indoors if the weather is foul.
Maybe at some point do a long relaxing bath or full facial/nails/deep condition hair etc.
If DM won't let you go to her for 25th, can you meet up with her and DB on 26th or 27th? (And I presume she would have a phonecall on 25th for you to say Hi to everyone even if not zoom/skype).
The same for DF - could you meet him for a while on a later day? Or visit DUncle on 26th if DF has left, so you can still see DGP etc?
Is there a friend you can go for a walk with 1 of the days?
And try not to dread it as that makes it much easier for the demons to take hold. I know it's trite, but look forward to it by having various things that are treats to you (whatever that means to you) that you will be enjoying spread out over the time. So you have small moments to look forward to enjoying instead of 1 big hole while DD is away. They don't need to be elaborate or expensive, just things that cheer you up.