The difference between discipline and self-discipline
When you think of discipline, what comes to mind? The military? School children in class doing what the teacher says? A child following their parents orders?
Those are the traditional concepts of discipline that are well-ingrained in most people's minds. But I'd like to argue that not only is that not true discipline, it's the opposite.
The Discipline of Authority
It's easy to fall into the trap of thinking that following orders makes you disciplined. Yet if discipline can be considered a virtue, obedience to an authority that screams at you, manipulates you, and threatens you, is anything but virtuous. Following orders because they've come from an authority figure is slavery and avoidance of responsibility.
Every human being has a responsibility to make judgements about what is good and bad. Nobody can make that decision for you. So if an order from above is barked at you by an authority figure (whether your parent, a teacher, or a commanding officer), blind obedience is abnegation of one of your fundamental responsibilities as a human being: the responsibility of morality and discernment. Something you must not delegate to someone else.
When soldiers and police officers hide behind their uniform and badge and say "I'm just doing my job", that's a perfect example of avoidance of responsibility. It doesn't matter what clothes you wear, you're still responsible for your actions.
Obedience to authority is often a short-circuit in the follower's brain. Not only are you taught to follow commands without discernment, you're never taught initiative and how to formulate your own goals. "What's good for you? What do you want?"
My parents never asked me that, and neither did any of my teachers at school. I suspect military officers aren't going to ask you that either. The fact is, nobody will care about your interests and goals in life as much as you will.
When people grow up and leave a controlling home, or graduate from a school in which their every minute was regulated, or worse, leave the military, their life is often a chaotic mess. Rather than learning discipline, all they've learned is how to follow orders. They cannot discern right from wrong, because that's not their job as a follower and would have interfered with following orders, and they cannot set goals for themselves. If they figure out how to do these two things, they're then hampered by the lack of a screaming voice in their ear telling them to follow through on their goals. Sometimes, they simulate that for themselves with self-judgment and internal bullying. More often than not, they find a controlling spouse or boss who will tell them how to live life.
And live they will, but they will live their life for someone else, and so it won't really be their life. They'll just be a sad, mindless automaton, without responsibility and fulfillment. They'll wonder why they lack "motivation" to follow through on goals they likely never even set for themselves. They'll never really be a complete, actualized human.
A few will break out of that, but it will require hard work. As someone once said, it's much easier to build up healthy children than to repair broken adults.
Self-discipline
The alternative is disciplining yourself. It's setting your own goals (initiative), deciding how to achieve them in an effective and moral way (discernment and moral judgment), and finally, following through (discipline). This is a vital set of skills that will be used every single day for the rest of your life, and the earlier you start building these up, the better. Ideally, parents should start teaching their kids this in early childhood, and those children should avoid going to any typical schools in which orders are barked at them by teachers, and should certainly avoid enlisting in the military.
Self-discipline is a habit that you have to practice every day, until setting goals and achieving them becomes a daily part of your life.
There's more to self-discipline than that though. You have to learn to prioritize and focus on the most important goals, lest you become overwhelmed. You have to set aside meaningless goals that are just busy-work because otherwise you'll start to wonder why you're even doing what you're doing. You have to set achievable goals or else you'll feel like a failure. All of this takes practice, and if, like me, you didn't learn these skills as a child, there is hope yet. And if you want to know what you want from life and then achieve it, there's no alternative. Blind obedience to others will only get you fulfilling their goals.