They never see their positive influence or successes. They are crippled by their own unfounded sense of failure. Build on that and do more of it. After every lesson ask yourself what you did poorly. We cannot correct mistakes or reinforce weaknesses if we don't recognize them or admit them.
Some teachers never look for their faults so they go uncorrected.
You and I must change if we are going to grow. A teacher is worth nothing who is not a student first. Study the Bible. Study the Scriptures first before going to commentaries, manuals, or lesson books. Whatever insight you receive during this period of study will be yours , and not simply a parroting of what someone else knows.