"The Man in the Arena" is the title of a speech given by President Teddy Roosevelt in Paris on 23 April, 1910.  It was reprinted in his book Citizenship in a Republic.

I became acquainted with the quote when one of my Marines spotted it in the Stars and Stripes.  He cut it out and gave it to me during a hard week during deployment when nothing seemed to be going right.

I've carried it in my wallet ever since and it has served me well on many occasion.

"It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat."

The Full Speech