Shun distemper. Seneca, Epistles 2.18.14-15

How do we handle anger? Check it, always. Control the mind, not the world, so that you avoid unnecessary fevers. 

Sed iam incipiamus epistulam complicare. Prius inquis redde quod debes. Delegabo te ad Epicurum, ab illo fiet numeratio: immodica ira gignit insaniam. Hoc quam verum sit necesse est scias, cum habueris et servum et inimicum. In omnes personas hic exardescit affectus; tam ex amore nascitur quam ex odio, non minus inter seria quam inter lusus et iocos; nec interest ex quam magna causa nascatur sed in qualem perveniat animum. Sic ignis non refert quam magnus sed quo incidat; nam etiam maximum solida non receperunt, rursus arida et corripi facilia scintillam quoque fovent usque in incendium. Ita est, mi Lucili: ingentis irae exitus furor est, et ideo ira vitanda est non moderationis causa sed sanitatis. Vale.

But it is time for us to begin wrapping this letter up. “First, pay off your debt!” you say. I will refer you to Epicurus, who has your coin: “Left unchecked, anger gives birth to insanity.” You must know the truth of this saying well, since your life includes servants and enemies. Anger is always kindling in all people. It arises as much from love as hatred, and infects us in serious affairs no less readily than among games and jokes. What matters in its birth is not any external circumstance, but the quality of the mind that it invades. Anger is like fire in this: it does not matter how great it is, but where it strikes. Dense matter will not receive a flame, though it blazes as high and hot as can be, while dry kindling nurtures even a tiny spark until it becomes a roaring fire. That is the way things are, Lucilius. The outcome of a mighty anger is a raving fever, and so anger should be shunned not merely for the sake of temperance, but for the sake of our health. Farewell.