Marcus Aurelius
“The universe is change; life is opinion.” Paraphrased from Meditations, book 4, section 3. The point: how we experience our lives depends heavily on our judgment of things.
“Do not waste the rest of your life thinking about other people.” Paraphrased from Meditations, book 3, section 4. A reminder to direct your time at what matters.
Seneca
“It is not that we have little time, but that we waste a lot of it.” Paraphrased from “On the Shortness of Life”, chapter 1. Seneca advises spending time deliberately.
“We suffer more often in imagination than in reality.” Paraphrased from the Letters to Lucilius, Letter 13. Many worries never come to pass.
Epictetus
“People are disturbed not by things, but by the views they take of them.” Paraphrased from the Enchiridion, chapter 5. Probably the most widely quoted stoic sentence.
“Some things are in our power, others are not.” Paraphrased from the opening of the Enchiridion, chapter 1, and the core of stoic teaching.