I can understand as I was once a "scatterbrained pianist".
It turned out that it was simply (or not so simply) that I had a horrible (relative to what it is now) technique. (I was overemphasizing certain movements and was not interdependently coordinated.) This meant that learning pieces took too much time and required me to "polish shiny objects". I learned the beginning pages of many pieces but could never learn the rest of it because i didn't want to make the effort as the effort was already too much. I also got bored with the pieces due to the "polishing".
See, playing the piano was an intense labor, even though I didn't realize it at the time. I didn't like to practice but liked to make the effort of practicing. I counted practice in terms of hours instead of experimenting with the correct coordination of my body to play with maximum ease. Piano playing was just playing the correct notes at the right time with the right dynamics with the right speed. How I did it wasn't part of the equation.
This is just what RC experienced though I labored through it for more than 4 years and only recently learned to interdependently coordinate my body. Worst 4 years of my piano study going through 3 teachers, one of which promised that "I have no technical difficulties and none of my students have technical difficulties." (This turned out to be untrue and though he was a famous pianist once, he too, had technical crutches which prohibited him from playing music that was beyond his abilty.)
Realizing this fault turned out to be one of the most revealing things that I have ever learned about myself. It alone explained why I would jump from piece to piece never learning them entirely. Once I started correcting this fault, I was almost immediately able to learn entire pieces because there was no need to "polish shiny objects"; instead I was polishing the movements of my body. The even greater consequence was that pieces which were once impossible became easy. I looked at pieces not about how easy or difficult they were but how I had to coordinate my body to make all of it easy.
Also, how do I re-approach a "half-learned" piece?
These were some of the most joyous occasions I have ever had with re-learning poorly played pieces. When I learned these pieces, even though they sounded well to my teachers, they were difficult to play and always required practicing. But going back to see if what I learned could transform these pieces caused fear; could I play them without the laboring I had?
Oh my god...
Faster, more accurate, more control both pianistically and musically...
Easy.
I couldn't believe that certain passages I practiced hours and hours until my fingers, wrists, and arms ached suddenly became the easiest things I had ever done at the piano. I got up with a big smile on my face within minutes. Oh my god... I had no one to share my joy with.
The confidence that came with being able to do away with "hard", "difficult", and even "impossible"... It was easy.
Learning became easy. I didn't have to struggle with the first page or any page. I knew by looking at the score if I could coordinate my body to play it or not. If I wasn't sure, I went to the piano to learn the coordination. See, once I figured out how to make the impossible easy, everything became easy, even getting past page 1.