A writer friend sent me the following quote:
If you don’t feel that you are possibly on the edge of humiliating yourself, of losing control of the whole thing, then probably what you are doing isn’t very vital. If you don’t feel like you are writing somewhat over your head, why do it? If you don’t have some doubt of your authority to tell this story, then you are not trying to tell enough.
~ John Irving
I might add that this is just as applicable to living in general as to writing in particular.