YOU HAVE POWER OVER YOUR MIND, NOT OUTSIDE EVENTS.
What it actually means:
You can't control what happened, but you control what you do with it
Real-life moment:
You're giving a presentation and the tech fails you. Slides won't load, mic cuts out, whole thing's a mess. You can't fix the technology in that moment. You CAN decide if you're going to freeze and apologize 17 times, or take a breath and say "okay, let's do this the old-fashioned way" and just.. keep going.