Stoicism isn't anti-improvement. Epictetus was an abused slaved, then philosophical leader. Seneca was rich and was tutor and adviser to the emperor. Marcus Aurelius was the emperor.

Yes, Stoicism teaches you that you don't have (at least ultimate) control over the world. But it does emphasize that you have control over yourself. You internalize your goals: Instead of "winning the tennis championship" it's "be the best player I can possibly be despite circumstance".