First of all, thanks for taking the time to write up the detailed answer to my question. While reading it, I realized that the OP was talking about fear of not fitting in DURING the process of achieving success, not after. That makes more sense and your position made me aware of that.

Regarding your fear of the unknown, I would say (as would anybody) that this is only natural. The best thing we can do is constantly remind ourselves of something that encourages us. I look to Frost's beautiful poem "The Road Not Taken", especially the ending stanza:

  I shall be telling this with a sigh
  Somewhere ages and ages hence:
  Two roads diverged in a wood and I -
  I took the one less traveled by,
  And that has made all the difference.