Those things that really terrify you
I've been reflecting on how we all have fears but there are none worse than those overwhelming ones, the ones we just can't control. These are the fears that, no matter that you "know rationally" there's nothing to fear, every time you think, see or experience "it"... whatever "it" (the feared object or experience) is for you, your subconscious mind overwhelms your rational thoughts and you can't help freaking out.
Maybe for you it's getting a needle, taking an elevator or flying in airplanes, but your brother has that fear of heights, and your best friend runs screaming from dogs as small as Chihuahuas. It seems like none of you can control that fear when it comes on.
Your heart starts to pound, you feel like you can't breathe, you want to scream but your mouth has gone dry and your knees seem like they're going to give out on you. When the feared object/experience is finally gone/over you find you're still shaking and you feel nauseous.