It Might Have Been by Ella Wheeler Wilcox.

We will be what

we could be. Do not say,
"It might have been, had not or that, or this."
No fate can keep us from the chosen way;
He only might who is.

We will do what we could do. Do not dream
Chance leaves a hero, all uncrowned to grieve.
I hold, all men are greatly what they seem;
He does who could achieve.

We will climb where we could climb. Tell me not
Of adverse storms that kept thee from the height.
What eagle ever missed the peak he sought?
He always climbs who might.

I do not like the phrase, "It might have been!"
It lacks all force, and life's best truths perverts:
For I believe we have, and reach, and win,
Whatever our deserts.