The best intructors that I had in GA was a crusty old
pr!ck named Col Griffin and my first boss, Bob Courtenay.
The first was an exWW2 Mosquito pilot who went to ANA and retired from the airline with 23,000 hrs of which 32 hrs were in single engined aircraft. He then did a Gr 3 IR and finished his career as a SE instructor. Col instructed me for some of my UPPL and also my CPL. He could be a rather blunt chap at times, he certainly didn't mince words but if you got over yourself and listened to what he was saying then you learned. The most valuable thing that I learned from him was that you needed a certain attitude towards your flying. Years later, I ran into him and told him with some pride that I was working as a B747 FO. He said to me, "Anthill, I knew you could do it because you are a survivor". He would know. It is the most flattering thing anyone has ever said to me.
Bob Courtenay was my first Boss. From Day 1 of his employ, he furthered the concept of having an 'attitude' towards your work and in a commercial sense. On my first day, I showed at the appointed time of 0800. He said that this made me neither late nor on-time- he expected that I would not ever be late for work and that the only way not to be late was to be early! He also used to knock back charter work if he couldn't do the job to 100% of the clients expectations. For example, if the charter needed to depart at 1400 and was to use an aircraft that was on another job arriving back at, say, 1200. He would say "can't do it". This ensured an optimum level of reliability to clients. It was notable that he had the only pure charter and air work business at MB; all of the other companies that did charter were also flying schools or RPT operators.
The key word here is "attitude". An instructor can try to impart the nuts and bolt of technique. A true mentor will go beyond that and show you the way to approach something. A mentor will cause you to change your outlook and perception in a constructive and everlasting way.