"It Might Have Been" by Ella Wheeler Wilcox.

We will be what

we could be. Do not say,
"It might have been, had not this, 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 force, and life's best truths:
For I believe we have, and reach, and win,
Whatever our deserts.