For different fields, having self-discipline means different things. Personal development fans consider it the ability to change habits. Musicians consider it the ability to get up and practice every day, so they never fall behind in their level of skill and muscle memory. One angle I'd like to look at self-discipline from, while not entirely containing the concept and all that it entails, can certainly be helpful: self-discipline is the power to act on ideas. It is the ability to take things from thoughts and realize them through actions and real results. Learning or creating a perfect system for processing, acting on and organizing email efficiently is not going to be worth a damn until you can force yourself to do it whenever you check your email.