Attaining the goal of Yoga, knowing ourselves on all levels—physical, mental, and spiritual—and attaining mastery of myself is more difficult than one might think. I believe I am one way and not another way, but I am mistaken because I can't really see myself clearly.
Without realizing it, I am wearing invisible blinders created by my personality and my ego. Looking in the mirror doesn't help because I only see what my ego allows. This is where a teacher comes in.
A true teacher, who I trust because s/he is selfless and clear minded, can help me become aware of the parts of my personality I am not aware of. They do this, often, by setting up circumstances in a way where we are forced to confront ourselves, and notice the parts of ourselves of which we were not previously conscious.
This process can be uncomfortable, but it is an essential step on the road to Self-realization.