Vacuuming.
Impossible activity: you have to focus on cleaning while not knocking things over. There are too many details, there are angles and momentum, you have to compute the Newtonian physics of dimensions, it can't be done, it's overwhelming, your mind wants to shut down, you long for sleep.
Any movement pushes you to tears. Stand up: cry. Go to the kitchen: cry. Sit in one place, immobile: ok. Change positions: cry.
So that vacuuming is its own dark anguish, where every moment is movement, and tears fall to the sound of "Our House", absurdly but jauntily whistled.