That's something I think about a lot. Refining technique from the basics, and kind of "rebuilding" my technique for the better. What kind of exercises did you do? Or perhaps what is not as important as how. Did you drop all the other advanced repertoire just to focus getting everything right?
Well, I did it with out a teacher (don't do that, takes way longer and you screw up everything repeatedly) so I didn't quite have the discipline to drop big repertoire.
I did however re-evaluate my approach to repertoire in general regarding what I considered to be my level.
So for starters, as well as looking at new pieces at the level I began playing every single piece of music I could get my hands on at what you might consider an easy level (like grade 1-3 standard to begin with) and gradually progressing through levels but covering hundreds of works in each, not just 5-10. I didn't really learn them perfectly, or play them over and over but I was applying lots of new ideas about how to play in these easier contexts. As the grades got a bit higher I put more time into the "standard" works at the level - so where during my exam years I learnt a prelude and fugue by bach, now I went back and thoroughly covered inventions and sinfonias completely for example.
I also raised the bar on performance standard significantly, not just in terms of what I might gain from listening to a recording from myself but in terms of how I felt and thought during the performance as well.
I formulated VERY detailed "sound images" about how I wanted to play and always pushed to get closer to these.
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Aside from piece work, I didn't really hit exercises as written by anyone, though I have played through lots of them to see what the authors thought in regard to what was important to develop. "Exercise" in itself is the wrong approach, its not practice these notes and see an improvement after however many weeks. The notes are designed (if they are well authored) to force physical problems that can't be overcome without using the correct physical approach so its much more about taking a set of notes and thinking about and testing how to do it so that it becomes easy. Once you find the easy (physical) you're at a point where easy is not easy, because mentally its VERY hard. So you are tasked with making the physically easy, mentally easy too.
To do this you need to get well versed in how your brain learns, and become very sensitive to the signals your body provides. So you start out by play one note, and focusing intently on what it feels like to do that - and you need to explore gradually play a signal note hundreds if not thousands of times observing first what your fingers feel like. If you change what your finger does (level of curvature, force applied, which parts move etc.) how does that impact what it feels like.. then how do all these changes impact what it feels like in your wrist, upper arms, shoulders etc.. all these body parts are explored mentally, and in great depth.
Once you are acquainted with a single note, on each different finger, you start on note transitions between fingers - and you're going to be here for a long time because there is a great deal of combinations. Be a bit systematic. 1-2, 1-3, 1-4, 1-5, 2-3, 2-4, 2-5, 3-4 etc. and in reverse, 2-1, 3-1, 4-1, 5-1, 3-2 etc. But remember there's now different keyboard configurations, like semitones (of which there are quite a few variants - white-white, white-black, black-white OR (in more depth), white-black, white-black, [black, white]black-white, [white-white]black-white).. plus this stuff in-front of you or at the extremities of the instrument.
If you explore physical sensations in all these different configurations you will learn a great deal about what can be sensed at the piano and the difference between your fingers and the general topography of the keyboard.
But the critical is not so much doing as your told by a me or a teacher, but to honestly think about what there is to deal with in fine detail and experiment with it.
Learn a bit about your anatomy, and what different functions are open to you in a physical sense (how can i move differently?) and test the possibilities in a musical context.
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Above all, everything I discovered during my 'experiments' was applied to real music with conscious thought. And I observed and emulated a lot of different pianists physically, to get a sense of different movements they used, and I read HEAPS of literature on piano technique.