You should become the master of your mind, not let your mind master you.

Buddhism Day by Day

Wisdom for Modern Life by Daisaku Ikeda
Tuesday, January 29, 2013:

 

It may seem perfectly all right to put ourselves and our own wishes first, to simply follow the dictates of our emotions and cravings, but the truth is that there is nothing more unreliable than our own mind. Life doesn't always go like clockwork and things will not necessarily turn out as we hope or plan. Consequently, Nichiren frequently stressed: "You should become the master of your mind, not let your mind master you." We mustn't allow ourselves to be ruled by a self-centered mind. Rather, we have to discipline our mind and gain mastery over it.