Full Idea
Temperance and courage are destroyed by excess and deficiency and preserved by the mean.
Clarification
The 'mean' is the middle route between two opposed vices
Gist of Idea
Virtues are destroyed by the excess and preserved by the mean
Source
Aristotle (Nicomachean Ethics [c.334 BCE], 1104a23)
Book Reference
Aristotle: 'Ethics (Nicomachean)', ed/tr. ThomsonJ A K/TredennickH [Penguin 1976], p.94
A Reaction
It sounds as if drifting off into an excess, like binge drinking, is not just having a bad day, but actually 'destroys' the virtue. Presumably it permanently diminishes the good habit.