We've all heard the infamous saying: "One should invest at least 10,000 hours of practice into a skill in order to become a master at it." Many people however, are quick to overlook the nuances in this saying, which might be exactly what is holding you back.
It looks like people are quick to wave away some of the most important aspects of practice, in order to make themselves feel good about the duration thereof. Many students sacrifice so much time hitting piano keys and sliding down a cello's fingerboard, paying little attention to the sound they produce while being satisfied way too easily.
Though, let's be honest, we've all been there, that's why you'll find it to be so relatable. It is something we shouldn't be particularly ashamed of but rather have the awareness to call ourselves out on it, by understanding when we are doing it. It doesn't even necessarily have to be procrastination. There are just methods of practice that don't tingle our brains in the way that resonates with it most effectively.
Learning through movement
The cerebellum is the brain part controlling motor skills. It takes up only one-tenth of the brain by volume but contains near a staggering 50% of all its neurons (Ivry & Fiez, 2000). When we're practicing, we are basically working out this dense neuron package, which may very well be the most complex part of the brain.
Fundamentally, we provide our brain with knowledge which it stores by strengthening brain cell connections. In the case of practicing, the knowledge we input are the movements with which we want to produce the sound we strive to hear. This is a direct explanation as to why the part of the brain that processes movement is exactly the same part that processes learning; we are reproducing the movements we once learned, refined by years of cell strengthening. That begs the question as to why the movement itself isn't sufficient enough when it comes to practicing on your instrument.
The answer on that question eventually comes down to the way in which those brain cells strengthen.
You may or may not have noticed it yet but we predict our movements before we carry them out, which makes us control these movements better (Flanagan, Vetter, Johansson, & Wolpert, 2003). To put it into other words, every movement we carry out during a practice session is preceded by lightning fast brain (thought) processes that analyze variables, predict outcomes, execute movements etc.
From this, we can draw the conclusion that the more focused you are during your practice session, the more these processes present themselves by facilitating chemicals called growth factors which strengthen and form these brain cell connections.
In other words; Congratulations! You have now achieved learning. But is that all there is to it? No, of course not, it's science after all.
According to Etienne van der Walt, neurologist and expert in cognitive neuroscience, optimal sleep, specific exercise routines and rhythm are some key factors in enhancing the amount of growth factors. This means that repetitive practice (take a scale for example), isn't necessarily a bad thing. Most probably, you wouldn't be able to play any piece without your motor movements helping you out which you've acquired through repetitive practice. You definitely shouldn't expect to play any piece by Chopin, Rachmaninoff or Liszt without it.
However, this saying is key to our answer: "Insanity is doing the same thing over and over again and expecting different results." If you're striving to improve your skills, repetitive practice can be your worst enemy. The main goal of practicing is to get rid of your flaws and make room for improvement.
Focusing on one thing can distract you from all the other errors, strengthening those faulty connections you want to get rid of by continuing to make those errors. In order to prevent this, you should make slow practice your best friend.
What seems to confirm this, is that changes in neural connections don't seem to occur when you aren't focused on the task at hand. Many studies conclude that a fundamental prerequisite for changing these cell connections is active engagement. I wouldn't expect you to learn much from your mathematics teacher, while you're browsing through Instagram watching memes. Although this might seem a little on the nose, it really confirms that while fiddling away might be more fun than slow practice, you should always strive for an efficient balance.
And no, it being boring isn't a valid excuse.
Don't underestimate motivation either. While you might think that they're just some emotions your brain ignores when it comes to getting better, motivation plays a very big role in how your memory interacts with given information. The brain activates different parts of your memory based on different emotional contexts which means that when you are eager to learn a piece, your brain is too.
According to the primacy-renecy effect, your brain has a tendency to store the first and the last part of, in this case a practice session, best. It might seem a bit abstract, so here is an example. If you have practiced today (which I sure hope is the case), there is a fair chance that you will be able to recall what, and how you practiced at the start and at the end of your practice session. You will most probably remember this in more detail than those two hour in between.
Make use of this given by practicing what you think is worth remembering the most at the start and at the end of each practice session. You can maximize these benefits by spreading out your practice sessions during the whole day instead of cramping it all into 3 hours straight.