The secret is: Your head lies. If you were to draw what your head says it would be wrong in so many ways that it would hurt. Your brain can get a awfull idea thats missing 90% of everything cool abd it could still seem perfect. You cant draw it because its not real, its just a illusion that stops you from second guessing yourself. If there would be no such mechanism in you brain you couldnt get anything done.

The real secret is that even the best of artists keep making many revisions, thumbnails sketches and so on. They do most often not get it right on one go. Unless they are copying what they allready see in real lofe in front of them. The work refines what you have.

In this process your brain does get better and you will eventually recognize what is truly there and what is not. But by then it will never be the same again. You still need to do revisions since you can not keep it in your brain in one go.