The way to write is to write 2.5 hours in the morning and do things that strengthen you the rest of the time.
But those are just practicalities. At a deeper level, you have to fill the project with yourself, as Proust might have said, and pull it onto your ground.
I find myself unable to work efficiently on Vallejo because I withdraw. I withdraw because I feel I am being crushed by the mountain of work on him that I am not interested in, but should theoretically control and address when I would much rather read him alongside other things. Outside as well are the shouting voices of various supervisors telling me to write more and faster (from an era when in fact I was writing more and faster than they, I must point out).
Now that I see the reason my mind seems to scramble is withdrawal, I may be able to improve the situation. The other thing I notice is that I do not seem to think Vallejo projects are my friends. Vallejo materials only seem friendly when I look at them as an amateur or fellow traveler. I find I must keep reminding myself that even at a professional level, they are my friends. This is somewhat odd, I find, but it is true.