Most entrepreneurs underestimate the difficulty and challenges that they'll face when they start something and if they knew how difficult it would be at the outset they wouldn't have started. Similarly, you seem to be assuming that you start with a great idea and you move in a straight line to its realization, however ideas evolve and where you end up may not be where you start. Maybe you just need to get started on something, and only by working thought the problem will you then discover the great idea.