Personal discipline is NOT sacrificing personal freedoms or forcing yourself to do something you don't want to do. That's basically a form of extortion, which demoralizes and drains energy.
Discipline's true meaning is found in its root word--disciple--which means follower. Discipline is the byproduct of deliberately choosing a path to follow.
The prerequisite to discipline is always getting clear about the destination and why it is important to us. It requires slowing down long enough to decide where we most want to end up. This clarity gives direction and allows us to "see" each step as taking us closer to what we most want, or farther away. Then, the choice is easy--not forced. Energizing, not draining.
Need more discipline in an area of your life? Try booking an appointment with yourself. Examine the path you're on and look ahead to where it will naturally take you--your health, your relationships, your finances, whatever is important to you. What do you see? What do you most want to see? Why is it important to you?
Do the hard work of answering those types of questions and discipline will be the least of your problems.