“It is precisely in times of immunity from care that the soul should toughen itself beforehand for occasions of greater stress, and it is while fortune is kind that it should fortify itself against her violence.” – Seneca the Younger

Practice pain and when it comes it will be gentler. If you have practiced the worst you can imagine enduring (without long-term damage to self), anything you can endure short of that becomes far less terrible.

Moral letters to Licinius – Letter 18. Found by Tim Feriss.