Some say that imitating the actions and thoughts of a mentor actually changes one's character and habits. Self-discipline is not something that is easily acquired unless it is developed in childhood, but it can be learned with practice.
Which biography inspired you to work on self-discipline because you realized that a person's success is directly related to self-discipline?
Reading David Goggins' biography, I realized how self-discipline is not an innate gift, but a choice every day. He went from an insecure guy with a lot of problems to one of the most resilient people in the world. The way he overcame himself inspires not just to talk about discipline, but to actually get up in the morning and do what you promised yourself.