The quitter by Robert W. Service.

It’s the plugging away that will win you the day,
So don't be a piker, old pard!
Just draw on your grit, it’s so easy to quit.
It’s the keeping-your chin-up that’s hard.
It’s easy to cry that you're beaten — and die;
It’s easy to crawfish and crawl;
But to fight and to fight when hope’s out of sight —
Why that’s the best game of them all!
And though you come out of each gruelling bout,
All broken and battered and scarred,
Just have one more try — it’s dead easy to die,
It’s the keeping-on-living that’s hard.