Practice
Practice makes perfect, as the saying goes. But is there a right way to practice such that mastery is achieved faster, or conversely, is there a wrong way to practice, such that the time invested is actually wasted? A novice cannot know.
I believe that the quantity (or time) of practice is the most important factor when trying to learn something. But I also hold the belief that practicing with appropriate methodologies can be vastly more efficient. Take learning a new language as example. There are systematic tools to help in the process, and the optimal strategy differs not only by language, but also by the current level of the learner.
Now I am practicing drawing. I don't have any sort of previous specialized education, and I am doubting my methods. Maybe it is Dunning–Kruger effect, or maybe there is something I can improve or alter in my practices.
Time will tell. But we don't always have the luxury.