Rejection. I spent too much of my early life as a pariah, that I over-compensated. Now, a lot of people love me. But I'm damn near obsessive when they don't. I try to correct it when I can, but man, it's awful.
EDIT: And not really a fear, but hospitals really eek me out. I get vibes and general auras from places, and hospitals are really wonky places. They also reek of death. The last time I went to one to visit a relative, I had to spend a cozy elevator ride next to a guy in a bed who had some severe pulmonary disease. When I figured out that the machines attempting to suck the mucus from his lungs in short, wet bursts (like a straw in the bottom of a glass) were not working fast enough, that I realized the man was drowning inside his own body. And on that day, I think I discovered the worst way to die.
*shudder*