From How to be a Stoic by Massimo Pigliucci:
The Stoic dichotomy of control applies throughout our lives. Let’s say you are up for a promotion at your job. You think it is reasonable that you will get it, given how many years you have been with the company, the quality of your performance reviews, and your good relationships with your coworkers and your boss. Suppose you are going to find out tomorrow whether you got the promotion or not. Adopting a Stoic approach will allow you to have a night of peaceful sleep beforehand and be ready in the morning to face whatever outcome comes your way, not with resignation but with confidence. Your confidence lies not in the outcome, however, for that is outside of your control. The outcome depends on too many variables, including the internal politics of your company, your boss’s personal sympathy (or not) toward you, and how much competition you may have from colleagues. No, your confidence lies in knowing that you did whatever was in your power to do, because that, and only that, is under your control. The universe doesn’t bow to your wishes, it does what it does; your boss, your coworkers, the shareholders of your company, your customers, and a number of other factors are part of the universe, so why would you expect them to do your bidding?
And even if you didn’t do whatever was in your power to do — even if you missed or didn’t take advantage of some opportunities to improve the likelihood of you getting a promotion — the fact that you missed those opportunities is no longer something in your control. Those opportunities are in the past now. Worrying about how having missed those opportunities in the past might impact something in the present — and losing a night of sleep over such worry — isn’t going to help anything!