To me, self-mastery means seeing myself for what I am, applying to myself the same standards I apply to others, overcoming behaviours and traits that I think are unacceptable or that I dislike in others, and through that becoming somebody I find admirable. Consistency and integrity, if you like. It's the quest to become the me that I want to be, not someone else, and not the me that it's easiest to be. In retrospect, I've got a twisted kind of gratitude for the times I've been forced to make unpleasant choices, because every time I manage to make one of those for the right reasons and not for the easy ones, I get to be who I want to be for a moment. I suppose it's kind of an Aristotelian approach to virtue.
I've recently realised that I've been approaching it wrong, by screaming at myself every time I fall short of that ideal. That's not self-discipline, it's abuse, and it doesn't help me. So my current project is working out what healthy self-discipline means for me (not an easy task in a world where the very concept of discipline seems to invoke Je.) I'm a work in progress.