I fear being common, average, pointless, weak, small, incompetent, etc... I have to be a magnificent, powerful, and brilliant human being. (Not in the eyes of other people, or in any way that is only measured by external success, but in my own opinion.)
So, in many ways, I'm afraid to fully try to reach my potential, because I might fall short and destroy my self-worth in the process. It sounds pathetic, but it's just like any other fear. When you have an unhealthy outlook, it controls you and holds you back, and when you have a healthy outlook, it's your greatest motivation.
I don't have any desire to live a life that seems small to me, but I don't want my "magnificence" to be achieved through pathetic means of a bloated ego, narcissism, or megalomania. I simply want to live the most amazing life I can imagine.
Things like rejection, loss, and pain are barely comparable to my main fear.