I don't think comparing science articles or even social science (clinical type) articles in terms of writing are similar to those in humanities and some of the social sciences like philosophy. They tend to be 8-12,000 words in length and rather than having one main idea (which you can bash out an intro/methods/results/conc fairly mechanically), they are often very intellectually complex and dependent on one person to deliver that rather than being the product of a bit of team-work.
I couldn't write an article like the one you are describing over Christmas, and it would probably have to wait til Easter next year for the main work to occur.
A couple of thoughts occur to me- is this to be 'REFable'? If so, it's worth doing, or if you will lose your job. If its just desirable/'everyone else has done one' then you don't need to do it quite so pressingly.
Also, I find doing a really small amount every day much better than leaving things then trying to immerse myself, so one hour a day starting after a week's Christmas break would be how I would approach this, do an hour most days except in the very intensive teaching time, and so by the time you really get to write (Easter) you have already structured the thing, gathered the references, started writing and so on rather than ignoring it and starting from scratch.
It is horses for courses though and everyone has a different writing style, I'm a plodder these days but I do get them out that way.
Also, you say your mental health is sometimes precarious, it sounds like you are worried about next term. I would come up with a plan of how to approach such an intensive term by ditching everything except the things that absolutely have to be done- and also have a plan for if you start to struggle (hopefully not but at least a way of alerting people if it starts to go wrong).
I don't have the ideal solution, it sounds like a lot of pressure. I probably should be pressuring myself over REF and perhaps my HoD is worried, but I can only do what I can do and keeping myself jogging along mentally and physically is important, one paper isn't going to make or break my career (or yours).