Mastering a skill means you have achieved a high level of competency in that skill area. Mastering yourself means you have achieved personal power.
Personal power is a state of mind achieved through the awareness and acceptance of the self. It means you know who you are, what your abilities are and continue to strive to reach a state of excellence.
Personal power means:
You are the most important person in your life.
You are the greatest "you" that has ever and will ever be.
You believe in yourself so much that you control the choices you make.
You believe in yourself so much that you control the actions that you take.
Mastering yourself means mastering all three dimensions of your life:
1) Spiritually you are able to create and maintain your passion for life.
2) Mentally you are able to create an image of your future, choose the steps that will lead you there and take action.
3) Physically you are able to perform at the necessary level that allows you to achieve your goals.
From: Performance: Your Guide to Personal & Professional Excellence by: Diane Matyas