If you have to decide every time you wake up, whether you'll get up early and go to the gym, you are forcing yourself to make that decision every single time. And with a limited amount of willpower, as you wrestle with that question, you will more often give in, then if you......Develop a habit.
Discipline is merely forcing yourself to create habits that will be right and beneficial for your life.
If you make a commitment to getting up early every morning, eventually it won't be difficult anymore, it will be routine. Once you have eliminated the opportunity for you to make a choice; When you aren’t giving yourself an option, then you can start to develop a habit.
As humans, we don’t really like change. So if you can change once, and then stick with it, then you can let routine take over instead of relying on your willpower and good decision making skills every time. As you work more with habits and less with willpower, you'll find you have the capacity to create a better life.