There are lot of different phobias out there, but I'm unable to find the one that relates to a "fear of not living up to ones potential".
The only word I could find is; Atychiphobia - fear of failure. Phobia list. That would seems similar to a 'fear of success', Jonah Complex. "The Jonah complex is the fear of success which prevents self-actualization, or the realization of one's potential. It is the fear of one's own greatness, the evasion of one's destiny, or the avoidance of exercising one's talents."