For the overconfident

Cicero, On Duties, Book I, p. 36

...Men whom success has made unbridled and overconfident should be led into the training-ring of reason and learning, so that they perceive the frailty of human affairs and the variability of fortune.

For those about to take charge of public affairs

Cicero, On Duties, Book I, p. 33

In general those who are about to take charge of public affairs should hold fast to Plato's two pieces of advice: first to fix their gaze so firmly on what is beneficial to the citizens that whatever they do, they do with that in mind, forgetful of their own advantage. Secondly, let them care for the whole body of the republic rather than protect one part and neglect the rest.

Doing splendidly if they have in them mere images of virtue

Cicero, On Duties, Book I, p.20

Since we do not live with men who are perfect and clearly wise, but with those who are doing splendidly if the have in them mere images of virtue, I think that we must understand this too: no one should be wholly neglected if any indication of virtue appears in him; moreover, one must particularly foster those who are most graced with the gentler virtues, modesty, restraint and [justice]...for a brave and great spirit in a man who is not perfect nor wise is generally too impetuous; but those other virtues seem rather to attach themselves to a good man.

All such apparent tragedies...

Epictetus, Enchiridion, Chapter 21:

Keep the prospect of death, exile and all such apparent tragedies before you every day - especially death - and you will never have an abject thought, or desire anything to excess.

Brevity of life

Marcus Aurelius, Meditations, Book Three, 10:

Forget everything else. Keep hold of this alone and remember it: Each of us lives only now, this brief instant.