I was a terrible student as a beginner and intermediate player. Actually, I was a terrible student even before I had a teacher. I tried teaching myself for 4 years. It was so bad, I couldn't even get the teacher in me to make the student in me to do what I was telling myself to do. After a very enthusiastic (but unproductive) start,and spending a lot of money buying DVD and Downloading courses from the internet. I gave up on the self-taught method.
I was determined to teach myself and told my friends, I’m teaching myself, which I was really proud off, I don't need to invest money into a teacher, I can do it as good on my own. Sure I improved some, but I also taught myself incorrectly and that really slowed down my progress. Of course I didn't realize this at first, but over time it became more and more obvious that I didn't really know what I was doing.
As a student, I thought I knew what was in my best interest to learn. I thought I should be the one to tell the teacher what to teach me. I thought it was the teacher's job to teach me whatever I wanted to know whenever I asked for it. And I also thought I had the right to tell him how and what to teach.
Now after having the opportunity to study with some truly great teachers, I still blamed the less than great teachers for my lack of greater progress in my early years. Soon after I began to teach others professionally, I finally realized that my prior lack of progress as a student was much more my own fault than that of my teachers. What is so obvious to me now, was inconceivable to me a decade ago. And this one simple fact caused more damage to my early musical growth than anything else.
An average teacher can help a good student much further and much faster than a great teacher can help a mediocre student.