I see the Stoics, especially Epictetus, as important forerunners of Korzybski's extensional ('fact'-oriented) methodology that he called "general semantics."
Indeed, Epictetus who influenced Albert Ellis (also a student of Korzybski's work), seems well worth studying today for his practical insights on extensional living. Epictetus after all was the one who said, "It is not the things themselves that disturb people, but their judgments on those things." Ellis' book How to Stubbornly Refuse to Make Yourself Miserable about Anything: Yes Anything! seems like a rather good description of Epictetus' thrust.
One on the Stoic path experiencing some 'unpleasantness', learns to say to it "You are just an impression and not what you seem to be." And then one can go further and ask, "Is it in my power to change this? Or is it outside my power?" Those questions and the distinctions they help to elicit, my friends, seem to me to provide a powerful method for not letting yourself become unduly miserable about the many inevitable slings and arrows of life. Simple, but not easy.
And what frame of 'mind' does it encourage but what Korzybski called "consciousness of abstracting," the sine qua non of what he sought to teach? Check out Corey Anton on Youtube for Reading Group, Epictetus #1