One of the biggest reasons folks never get started on a project is that they're afraid of not doing it "right." Consequently, they seldom even try. This "analysis paralysis" can afflict model railroads at all sorts of stages of development - from before birth (when a layout never gets started) to any point during its life. But when layout progress stops because you think you've gone too far down the wrong path, you may be suffering from a related affliction: "progress paralysis."
In either case, the remedy is the same: raising your comfort level with Doing It Over. Yes, the "Do Over" is something with which experienced model railroaders are all too familiar. They've come to realize that the only thing worse than doing something over is never doing it at all - and the regular progress of their layouts is testimony to their approach. Even one step back is progress if you eventually wind up with two steps forward. And you'll often get three or more steps forward if you've learned something new as a result.
Now, I don't consider myself an "experienced model railroader," having for far too long been content with armchair daydreaming (and my recent RR library weeding/reorganizing hasn't helped), but this layout project is teaching me - sometimes harshly - that I've got to get really comfortable with doing things over (or at least considering that possibility more often) if I'm going to be happy with the outcome.