A self-disciplined person can make themselves do what they know they ought to do, without someone standing over them. It is the adjective that matches self-discipline. You might be self-disciplined about sleep, money, sport, or study — it does not have to be all of life at once. Though actually English speakers use it as praise so often that it can sound like a personality you either have or you don’t. In real talk it is more like a habit in a certain area: someone can be self-disciplined at work and messy at home. It describes a person or their way of living. It is not a verb, so we do not say “I self-disciplined yesterday.” We say someone is self-disciplined, or they showed self-discipline.