When a child learns to walk, for instance, he gets immediate feedback as to whether he's doing something incorrectly. (That is, he falls down.) But often the feedback you get with a musical instrument isn't so easily interpreted. You might play the right notes in the correct order but at the same time use your muscles or your mind in a way that hinders progress down the road.
But let's assume you're getting good guidance and the technical elements are more or less in place. There's an unavoidable fundamental to playing: automatization. You simply don't have a choice but to make the physical elements of playing automatic.
What does this mean? Repetition.
The devil is in the details. They are important. But don't lose sight of the foundation of good practice. To become truly facile, you have to move to the level of the automatic. Watch any number of great musicians over time and you'll observe this simple common denominator: tireless, intelligent, repetition of the music.
Don't let details bog you down and sap your motivation. Keep it simple, over and over again.