I've have been on the exact journey you described - the idea that won't leave me alone (probably 6 years old now), Googled early on to see if anyone else had built it yet; found two existing companies pursuing the same thing, but in two different ways that don't quite match my own.
I've been sitting on the idea for the last two or three years (it still won't leave me alone), and it has now evolved significantly. Fortunately, due to a rare moment of foresight, I've been keeping a journal capturing thoughts around the idea as I've had them, so I've now got a really nice record of the original concept, iterations, evolutions and adjacencies. The best part though, is that I have a record of the excitement that this idea gives me (re-kindled every time I read it), which is a great pick-me-up when the self-doubt takes hold.
I am now entering my last months of employment as I ready myself to jump in and make it real!