What the expression is intended to mean, I think, is that there is a better and a worse element in the character of each individual, and that when the naturally better element controls the worse then the man is said to be "master of himself", as a term of praise. But when - as a result of bad upbringing or bad company one s better element is overpowered by the numerical superiority of one s worse impulses, then one is criticized for not being master of oneself and for lack of self control.
I did have a love for literature that overpowered my hatred of the people who taught it, and I think because I had no respect for the teachers, their attitude didn't poison the writing that I was discovering for myself.
When you find yourself overpowered, as it were, by melancholy, the best way is to go out and do something.