When I was young, five or so, I had two bad experiences. One was when my parents were trying to get me to learn to swim. The swim instructor was terrible- her plan was to, after one lesson, take us out to the deep end and have us swim to the shallow end. I, of course, sank like a stone. For a few terrifying moments I couldn't breathe. I was going to drown. I was going to die.
Luckily, I was rescued, and went home, and refused to take swim lessons for another several years (and rightfully so I think).
When I was a bit older than that, only a few months later I think, I woke up in the middle of the night unable to breathe, call out to my parents, to anything that required air. It was the most horrifying few seconds of my life. I lay perfectly still, waiting once more to die. Slowly my breath returned. I was taken to the hospital, where I stayed for two weeks. Every once in a while, the tightness in my chest returned, and I stared at the ceiling, prepared for death.
Because of this, I cannot handle the idea of being unable to breathe. I start to hyperventilate, (which only triggers me more), cry, and occasionally collapse to the floor.