The majority of people mistakenly judge external things as good, and therefore experience feelings of desire for things beyond their control, leading to frustration and suffering.
Stoic thought emanating from Epictetus, quoted from STOICISM and the Art of Happiness, page 34.
We should inward however to attain true happiness or εὐδαιμονία, “eudiamonia.”
The term “eudiamonia” really means to flourish or flourishing. Your life isn’t about just being happy but about flourishing, being your best, reaching ἀρετή or excellence. See Choice of Heracles below.