I know this one well! Why bother when the task will not get finished because my energy will run out before it's finished?
But, I seem to have found a way round this that usually works. Instead of expecting to get a task completed, I set myself a time limit to work on it (say an hour) and then at least I feel that I've accomplished something, which is better than nothing. O.K. the task may extend over several days but it does eventually get done. I also feel a bit more in control when I manage to think like this. (Which doesn't always happen - I do still get very black days)
I actually wish I'd tried this approach even before PD because I do feel a sense of achievement.
Don't know if this will help, but it may be worth a try.
Wishing you some energy,
skatergirl