Pessimists see their destiny and lives as out of their control, guided by fate or luck or "others," and that is how they "explain" events in their lives. "I seem to always have bad luck," is the common refrain of the pessimist, while the optimist explains a bad event with such words as, "Sometimes you're the windshield and sometimes you're the bug." They move on and grow from the experience, proving Nietzsche was at least right about one thing: That which doesn't kill us makes us stronger.
Optimists know if they are to control their destiny they need to put in the time and effort to create the greatest chance for a positive outcome in whatever challenge they may face. Got a new holster with a different draw pattern than the one before? Don't complain that the agency didn't give you time at work to do enough repetitions to make drawing from this new tool a habit; spend your own time doing rapid draw exercises from traditional positions as well as nontraditional ones, like lying on your back or sitting behind the wheel, until you can draw without any conscious thought whatsoever.
Optimists control the things they can and plan for the things they can't. For instance, you can't control a bad guy suddenly assaulting you. You can control how you stand, your reactionary gap, where your hands are held when talking to a subject, as well as your fitness and training level. This is the key difference between pessimists and optimists: Pessimists hope for the best, optimists train for the best.
From time to time I have had trainees ask me, why if I teach optimism do I teach them to visualize terrible events? I explain that true optimists don't avoid negative thoughts; they embrace them and use them to prepare to overcome the bad event.
I don't know if you are going to be in a shooting confrontation in the next week or not and neither do you. You don't control that. You do control whether or not you sit down, do your tactical breathing, and imagine yourself winning a sudden ambush. Terrible events happen to both optimists and pessimists, and just thinking "joy, joy happy thoughts" doesn't prepare either for these crises.