You are not alone. This happens to everyone. As you attain a higher level of skill, the learning curve gets steeper. This is the primary reason that the great virtuosi are vastly outnumbered by mediocre amateurs. Along the ascent, it is increasingly difficult to resist the urge to give up. It's a bit like the analogy of the person taking steps towards a wall, each step half the distance of the previous one. That person ever more slowly advances towards the goal, without ever actually reaching it. It's maddening.
Practicing would be a far less frustrating affair if we could easily measure our progress from one session to the next. Imagine if you had one of those "downloading" progress bars floating underneath you as you practiced, telling you how close you were to completing that new sonata or jazz tune! Although we are building neural connections with every repetition, it feels more like endless plateaus that suddenly break through to new levels, almost without warning. Enduring each plateau is a test of our commitment. The plateau is the perceived barrier between where we are, and where we are trying to go.
This barrier is perceived. It is not real. Improvement creeps in, almost too slowly to measure. This is especially dangerous when we are measuring the wrong things. When you find yourself at one of these practice plateaus, it is tempting to look at that those far-away goals and to feel overwhelmed. These are the precise moments where we decide what kind of artist we will be. Will we keep pushing up the hill, or simply surrender?
I have survived many of these moments with a simple strategy that works every time. Set a small, easily achievable goal. Make the goal tiny, something like "learn one measure." The smaller the goal, the better. Make it something that you could conceivably accomplish in 20 minutes or less. Rather than trying to learn an entire recital program, or a whole piece of music, focus on a very small chunk. This next part is very important, so read it twice. Read it twice.
Focus completely on the small amount of material and sustain your concentration on only that amount, without adding anything until the material becomes effortless.
Practice isn't magical. Every minute, every day, it always works in the same way. Your brain slowly enhances the circuits that you use the most. These circuits are built more efficiently when we repeat simple actions, over and over again. Although the big picture comes together too slowly to recognize, the tiny aspects are much easier to deal with. An attainable, short-term goal will bring a sense of accomplishment, and that reward will go a long way towards curing your practice plateau blues.