The greatest misfortune of ...

The greatest misfortune of all is not to be able to bear misfortune.
 Bias of Priene

Quotes from the same author

Be slow in considering, but resolute in action.
 Bias of Priene
It is better to decide a difference between enemies than friends, for one of our friends will certainly become an enemy and one of our enemies a friend.
 Bias of Priene