"You do what you are"

From Along Came a Spider, James Patterson

Here's another:

"In the clearing stands a boxer
And a fighter by his trade,
And he carries the reminders
Of every blow that's laid him low
And cut him 'til he cried out
In his anger and his shame,
"I am leaving, I am leaving,"
But the fighter still remains."

"The Boxer," Paul Simon

And this one:
http://www.poets.org/viewmedia.php/prmMID/16549

You gotta do what you gotta do. There is no other way. And there never was.