For me, this happens when I'm drawing/painting. I get to a point where I think it's perfect thus far, but if I do any more it will be ruined - BY ME. I have stacks and stacks of work 'incompleted'.
I must be so afraid of failing or proving myself 'untalented' that I stop before it's completed.
I wonder if being called STUPID all my childhood by my father has anything to do with it?
What say you, peanut gallery?