I've started many things in my life and failed/given up a number of times.
Failed Projects:
While I don't think all was lost of these endeavors, I learned a ton by trying; I wouldn't consider any of them a smashing success.
What stopped me? What killed my drive? Why did they fail?
Well, during each attempt to build up what I was doing, I would come across other sites doing something extraordinarily similar and get downright discouraged.
"Damn, these guys are way better than I'll ever be" or "I can't compete with this" or "if they are doing this what's the point of me talking about the same stuff?" or "these guys are venture-backed and have bajillions of dollars to do whatever they want with" etc.
It's all bullshit and an excuse not to create and do.
No more.
You are your own worst critic, and external validation is the devil.
I think it's a matter of having an internal vs. external locus of control.
Over focusing on external factors like what others are doing in your space, not getting as much attention as you'd like, or chasing what you "think" success is, are what bring you down.
Over and over again, it happens.
In this world of noise, it's hard to shut up the bullshit and ignore it's utter irrelevance to your mission.
You are building something; you shouldn't care what others are building.
You aren't building it for them. You are building it because you can, and it drives you. That feeling when a prototype works for the first time or a milestone is achieved, it keeps you going.
External validation, asking for permission, watching and wanting what other people have.
This is what is stopping you. Don't let it.