In Greek mythology Sisyphus was condemned to pushing an immense boulder up a hill, only to have it roll down and start again, for eternity. One of my favourite authors, who coincidentally would have turned 100 last week were he still alive, Albert Camus contended in the Myth of Sisyphus that a person who knows their purpose in life is content, therefore Sisyphus is actually truly happy. The size of the boulder, in our personal life, for each of us changes from week to week but nonetheless we all face the ongoing cyclic struggle of pushing and having to continually start over again.
Every movement and workout we do has a greater purpose than just finishing 'x' reps or shift 'y' weight. If you stall or start to get mentally beat down in a workout, step back and think of the bigger picture, the greater purpose....and start pushing that boulder again.
"The struggle itself is enough to fill a man's heart" Albert Camus