"Practice makes perfect!" This phrase is used so often in the world of music, but a more important phrase could very well be, "Practice, Technique, Performance". After all, the concept of practice making perfect is skipping some very crucial steps that need to be trodden in order to achieve success. How can you really know when practice is paying off? Once attaining a certain skill-set is within your grasp, your teacher (or yourself) pushes you headlong into a new concept or piece; and sometimes it is impossible to know whether or not you're making strides or wasting your time on futile exercises.
This thought can be very discouraging, but can also be avoided through reflection and continued perseverance forward, more and more towards the goal of anything from the universality of scales and arpeggios to the complexities of individual instruments or styles. Always make it important when in the throws of self-doubt to understand how far you have come through practice, and make it a point to efficiently practice every time you do. The BETTER you practice, the better you will feel about it (there are very bad practice habits, all of which diminish playing and overall psyche); and the better you feel about practicing, the more apt you are to set goals for yourself. Goals, after all, create the drive for improvement and are stepping-stones through which we can look back and quantify our growth.
If musicians or any other workers in any trade didn't possess the means to evaluate their current work to that of their former work, who's to say they wouldn't just abandon the work all together, leaving the arbitrary "perfect" for someone else to obtain? It is all too easy for the idea of perfection to get in the way of the investment that is necessary for one to feel accomplished in a realm of veterans. We often overlook the hard work and dedication that has lead successful individuals to their historic contributions in their field. It is also hard for aspiring artists to understand that the professionals also had their own failures. Once it is apparent that those failures drove said individuals toward success, however, it becomes even more of a resolution for the learner to also fail and feel the longing for accomplishment. This is why "practice, technique, performance", in that order, separates the good musicians from the ones that expect to become good musicians over night.
Through the time we spend becoming familiar with our instruments and the concepts that make them sound as they do, we understand that beyond the idea of perfection (which, when attributed to art is rather arbitrary...) is the idea of investment. The idea that, as human beings expressing ourselves through music we need to use the direction of the former masters to forge our own tools for success. Tools forged in time and frustration, refined in the fires of the confidence achieved through reflection, and finally used as our presentation of a culmination of time and effort. Practicing the techniques that make performance feasible is a never ending process that can either discourage or embolden an individual. Finding the way to the latter is much more enjoyable and rewarding.