Let's not get sidetracked on the details of the task, it would be the same if he had ordered you to cut down the tallest tree in the forest with a herring. What seems to me to be happening is:

1) You have been brattifying in a way that suggests that either
a) You want out of the relationship and are trying to piss him off till he dumps you. Since you've asked for help I'm guessing it's not that.
b) You feel neglected and undominated and want to prod him into taking charge of you hard. Not ideal behaviour for a sub, but sometimes you have to if your Dom's not holding up his end of the relationship. I'm guessing that the tough schedule you refer to may be part of the original problem: you want to be taken in hand and punished till your everyday stresses no longer matter?

2) He has failed to get the hint, or understand the real life concerns that are pushing you to demand more attention, and tried to carry things along at the level they were before, using the punishments that worked before. But they don't work now because they just add to the mundane problems by giving you an extra workload. And he has painted himself into a corner by giving you a task which you both know can't be done.

The best way out would be for him to decree that for your impossible task he will subsitute something short and sharp - a serious caning, a heavy enema, orgasm overload, whatever you use for punishments, but it should be close to your limit so that neither of you feel you are being let off. After which you will be forgiven if you behave yourself humbly herafter, but he will be keeping a tight rein on you. And then do keep that tight rein.

How you get that idea across to him, and make him understand that you have been acting up because you need tighter discipline, is another problem and depends a lot on your level of communication. I suggest a serious letter combining apology and appeal for help, and not saying too bluntly that he has been failing in his role as your Dom.

Keep us posted.