It's the visionary's job to convince them that we people only think the impossible things are impossible.
That's basically how I do my internal dialogue. I shoot down an idea of mine because it's too brittle, vague, and difficult. But I still want to build that something. So, I keep thinking and end up saying to myself, "Well, maybe I could do something that's like an ugly partial implementation, just leaving out the hardest things: it won't be what I want but I can write something that resembles it." And then I write the first prototype and end up having something here to play with. However, I still keep wanting more and maybe I get an insight that allows me, having first played with the first build, to make a better approach with a new set of tradeoffs but such that will get me closer to what I want. Gradually I approach what I want, possibly never quite reaching that point, but still getting closer and closer.