People can have a lot of different reasons for doing or not doing a certain thing. You eat because you're hungry, or you don't because you're sick, etc. While those types of causes and actions are geared towards sustaining your life, basically, we tend to treat our work in similar ways. I.E. "I want to get work as a musician, so I can eat. So I gotta practice." I usually say that people can rationalize any action they choose to take, no matter how crazy it may seem to the beholder. The main issue is taking that step to rationalize yourself into the action you wish to take.
Hopefully playing music is something you enjoy. If it's not, well, it's going to be hard to make yourself practice when you don't want to, but more on that later. If you like it, that's only a piece of the rationalizing puzzle. For me, liking or loving what I'm doing is an emotional motivator, which may or may not be strong enough to fight against being tired, or grumpy, or discouraged. So, I had to find a way of thinking that would counteract those variables when they appeared. Thinking about the process in a way that had nothing to do with how I felt, but where I wanted to go really helped me find healthy ways of responding to my discouragement or fatigue. If I was feeling like I wasn't progressing, I would look objectively at what I had accomplished during the day. Literally, any accomplishment, no matter how small, is a step forward in my mind now. So what I didn't learn the entire first page of a piece. Maybe I only learned the first 4 lines, that's progress.
I've practiced when I was exhausted, and probably shouldn't have, but somehow I made myself get out of bed, grab the sticks, and hit something, even for a few minutes. Honestly, that's from watching movies or seeing someone I respect do the same when they are really down and out. If you know the story of Rocky Balboa, you know how crazy his workout routine was to prepare for a match. The slogan of one of the films was, "no pain." Clearly he was in pain, but the mindset that they cultivated was that pain didn't matter, you keep going anyway. Of course, with a grain of salt here, but the spirit of it is what I'm getting at specifically. Also, my parents are extremely hard working, and I figured if they can work for hours and still come home and take care of us, I can at least get up out of bed and play stick control. What's the least I can do, even running on empty? Ask yourself that when you are exhausted, or just fed up.
Like anything, cultivating discipline is a habit you have to form, and I really believe that it starts with how you think. The previous scenarios are only two processes of thought, but for me they are the most important. Discouragement and fatigue were and continue to be my biggest foes in my career, especially now given my current health circumstances. In spite of that, I'm releasing a book, doing Facebook live streams, practicing for a concert, basically doing exactly what I've been doing and then some. How? It starts pretty small, like playing five minutes of stick control every day. Take five things like that a day, and you have a twenty-five minute practice session. Do it every day, and you have a practice routine. Consistency is the most important part in cultivating that muscle of discipline. If you let yourself lax on a day, or an exercise, you'll do that for two, or three, and eventually you'll lose a day, a week, etc. Start small, start where you are, and build to where you want to go.
With starting small it also can be encouraging to see yourself complete a task of that size. Again, combining multiple small tasks can result in one bigger task. Rather than trying to take on 4 hours of practice a day before you've even done stick control daily for five minutes, can result in failure and then discouragement, leading you to never attempt it again. Start with something you know you will be able to achieve, take pride in it, put all of your attention and drive into that task, enjoy the process, and do it every day. When you are ready, add another task until you reach your ultimate goal.
If you are like me, the feeling after completing a task is motivation enough to begin it. The fear of not being successful or completing the task deters me away from even beginning at all. You must go into it knowing, or convincing yourself, that you will complete the task. Even if it isn't where you want it to be in the end, there is always tomorrow to come back and try again. One day is not a reflection of the future or yourself as a musician. Really fight the urge to characterize it as such. It's only a small segment of the work that you've done and are going to be doing. Even the end result is only a small segment, because you still have so much to learn and improve on. Really embrace the fact that you will always be improving on something, and take it as not only a challenge but a relief, because you never have to be perfect at anything, you simply have to improve.