I like this a lot, except I'd be cautious about using "good" there. I know some anxious students who would feel like they were automatically bad at math because they couldn't do all that yet. Maybe "I have mastered the math concept when.
Since I'm going to be a high school math teacher, this could not apply to me anymore. I try to live by telling myself that any problem or situation I get myself into, there is always a solution. This goes for math as well.