John Glen, the first American astronaut to orbit the earth, spent nearly a day...

John Glen, the first American astronaut to orbit the earth, spent nearly a day in space still keeping his heart rate under a hundred beats per minute. That's a man not simply sitting at the controls but in control of his emotions. A man who had properly cultivated, what Tom Wolfe later called, "the Right Stuff."
But you...confront a client or a stranger on the streets and your heart is liable to burst out of your chest; or you are called on to address a crowd and your stomach crashes through the floor. It's time to realize that this is a luxury, an indulgence of our lesser self. In space, the difference between life and death lies in emotional regulations. Hitting the wrong button, reading the instrument panels incorrectly, engaging a sequence too early- none of these could have been afforded on a successful Apollo mission- the consequences were too great. Thus, the question for astronauts was not How skilled a pilot are you, but Can you keep an even strain? Can you fight the urge to panic and instead focus only on what you can change? On the task at hand?
Life is really no different. Obstacles make us emotional, but the only way we'll survive or overcome them is by keeping those emotions in check- if we can keep steady no matter what happens, no matter how much external events may fluctuate. The Greeks had a word for this: apatheia. It's the kind of calm equanimity that comes with the absence of irrational or extreme emotions. Not the loss of feeling altogether, just the loss of the harmful, unhelpful kind. Don't let the negativity in, don't let those emotions even get started. Just say: No, thank you. I can't afford to panic. This is the skill that must be cultivated- freedom from disturbance and perturbation- so you can focus your energy exclusively on solving problems, rather than reacting to them. p28-9