Calm under pressure

Thomas Jefferson wrote:

Nothing gives one person so much advantage over another as to remain always cool and unruffled under all circumstances.

I was recently looking into a Jewish spiritual practice called mussar, and the first desirable characteristic to be developed in that practice is to maintain one's equilibrium at all times, avoiding both highs and lows. This smacks of the Buddhist practice of detachment. There's also that poem, by Rudyard Kipling?, about keeping one's head while others are losing theirs.

I don't fly off the handle much, but do occasionally veer towards depression or anxiety. Which is why I take a prescription mood stabilizer.

If avoiding unpleasant mental states is not sufficient motivation to try to remain calm, perhaps it would be worth it to gain some advantage by thinking rationally while others are being governed by emotion.