This is an old one, but one of my favorites.:
Loss and Gain
When I compare
What I have lost with what I have gained,
What I have missed with what attained,
Little room do I find for pride.
I am aware
How many days have been idly spent;
How like an arrow the good intent
Has fallen short or been turned aside.
But who shall dare
To measure loss and gain in this wise?
Defeat may be victory in disguise;
The lowest ebb is the turn of the tide.