There is nothing, I hold, in the way of work
That a human being may not achieve
If he does not falter, or shrink, or shirk,
And more than all, if he will believe.

Believe in himself and the power behind
That stands like an aid on a dual ground,
With hope for the spirit and oil for the wound,
Ready to strengthen the arm or mind.

When the motive is right and the will is strong
There are no limits to human power;
For that great force back of us moves along
And takes us with it, in trial's hour.

And whatever the height you yearn to climb,
Tho' it never was trod by foot of man,
And no matter how steep - I say you can,
If you will be patient - and use your time.

Ella Wheeler Wilcox

http://www.poemhunter.com/poem/limitless/