My teacher of the past 14 years has been Russell Waddell. He has been teaching since the 1970s and has had a fair share of students.He started driving down from Dallas to Austin to come for an occasional visit about three years ago.
On one such visit he expressed a frustration that he has had about martial arts students over the years. Most want to be spoon fed martial arts. Most people come in 'brain neutral' and want all the answers given them.
Every time he came down I would barrage him with questions about how to make technique work...etc. He told me that he does not want to rob me of the experience of discovering how to make it work on my own. Instead he came down and taught seminar after seminar about how to solve problems, and how to think about Aikido. He granted me permission to think - something most martial arts teachers don't want. He told me that I had to become my own teacher.
So now I continue to train with many great teachers. However when it comes down to it, I am the shaper of my ethics, art and future. No single person can shape my own art more than I can. I do not depend on my teachers to solve my problems.
So the most important lesson I have been taught to date is to take responsibility for your own training.