I distinctly recall a particularly frustrating practice session during my first few weeks on the oboe. It was the day that I decided to practice upstairs in one of our smaller bedrooms. That was the day that it dawned on me that my sound was like that of a balloon whose air is being let out between your fingers. This was probably one of the only times I actually cried out of frustration. It was also the only time I considered giving up the oboe. I remember thinking how annoying practicing scales had been on the saxophone. Yet here I was wishing with all my might that I could only play a C major scale. Just one octave. At any tempo. But in tune. And without sounding like a flock of geese.
Last night I didn't have a lot of time to practice so I took out the Pares scales book I have and just played some stuff from there. I was trying to "lose myself" in the music and the feel of the scales in my fingers. I think it may have worked for a few seconds at least and then I went back to all the inner dialogue. At one point after finishing a long-ish exercise the practice session I described above came back into my mind. I couldn't help but smile widely. I had just finished a long exercise on the C major scale. It included lots of slurred passages, some staccato, high notes and low notes, some crazy register jumps, and many dynamic changes. It didn't come out perfectly but it was leagues beyond where I was four months ago. Not only can I do a C major scale now, I can do pretty much any scale (though my speed is inversely proportional to the number of accidentals in the key signature).
It's easy to focus on what you're doing wrong ALL the time. There's always going to be something that needs improvement. In my case everything still needs improvement. But you also can't lose sight of the things that have gotten better. Recognizing this motivates one to keep on plugging away. :-)