God damn this thread is depressing.
...my turn!
I almost drowned when I was, oh, ten or eleven maybe. It was at a pool party and I slipped into the deep side. The weirdest thing is, I still remember it well. When I hit the bottom, I pushed myself back up and tried splashing to keep myself up. I did that a couple of times, attempting to scream for help in process (obviously not a good idea) and even thinking to myself while it was going on that I was going to die. Finally somebody's dad saw me and got me out. Oddly enough, I still enjoy going to the pool. (Of course, I learned to keep myself afloat after that.)
A few years before that, I had these reoccurring nightmares that I think I got from watching Star Wars, specifically the part where Han Solo gets sealed in that slab. I don't remember the scene scaring me, but for the longest time, I kept having nightmares where I was pressed in between two steel walls. I was still alive, but I couldn't do anything, and it was just so weird. Also, the sandwiching walls were located behind my couch. No idea how or why, but needless to say, I stayed far away from that couch.
The most traumatizing thing, though, would have to be my depression, but that first started when I was in the eighth grade. I was hit with this terrible feeling, and I have no other way to describe it than that. I laid in my bed and cried for days, sleeping as much as I could to get away from it. I eventually went to the hospital, and I've been there a couple times more because of it. I'm still on meds today, but I'm doing all right now, fortunately.