Sisyphus was not alone. Ixion, Tantalus and Tityus were also punished in Hades.
They were all – in life – well known and successful men. Each was corrupt in some way, however, and paid for it in hell.
Ixion was bound to a fiery wheel which was destined to turn forever. Tantalus stood up to his neck in a pool of water but found that when he attempted to drink, the water would flee his lips. Fruit hung in branches close over his head, but as he raised his mouth towards it, breezes blew it out of his reach. Tityus the giant was pegged out on the ground – like Prometheus chained to his rock – while two vultures tore eternally at his liver.
But it is Sisyphus who is best known for his thankless and eternal task, inching a gigantic boulder up a steep hill, only to see it roll down to the bottom once more each time he reaches the summit.
My advice is this: if you feel that your writing is like that boulder, falling away to nothing as you come in sight of your goal, you are too close to it and you have lost perspective. Your writing does not – cannot – lose all merit in a moment of inattention due to a busy fortnight with the children or at work.
You must put it aside for a time, then find a way to come back to it refreshed – objective once more.