Returning to Ranjit's original question on how to deal with imposter syndrome, I agree that it often arises from self-critique, but often it's an exaggerated emotional response rather than something fully rational. It comes and goes for me. On one level, I try to find perspective. I speak a number of languages, and I remind myself that learning something complex like a language or the piano as an adult requires a lot of patience and even faith in the stepwise learning trajectory, and that skipping steps is counterproductive. For example, I had French classes throughout high school, and when I got to college, filled with ambition and egoism I tried to read the most difficult things and to obtain an erudite vocabulary, largely ignoring things like conversation and some of the subtleties of grammar, in pursuit of something flashier and more immediately impressive. When I started learning other languages without prior experience of them, I realized that I needed to be more methodical, more open, and more willing to really try and fail, and thereby gradually master the basic, less impressive things first. It's more of a slog that takes years, but it's turned out to be more effective for me, and much less frustrating.
My experience of the piano is fairly similar to that. For a number of years I tried things that were way beyond my technique. My playing was accordingly inconsistent, even for relatively simply works. In my head, with all the theory that I read, and all of the pieces I was familiar with, I seemed to be in a really discordant relation between awareness and execution. I've since humbled myself from my earlier arrogance or stubbornness, continuously working on technique and relaxation optimization, and it's easy to feel imposter syndrome, due to the gap between performance and awareness. But I remind myself of what the learning process is for other things. And with learning more methodically and patiently, I also understand that playing the piano has a lot of parameters by which I can trace my progress, from understanding musical logic and detail, to basic technique (scales, arpeggios, patterns, double notes, octaves, chords, rhythms, polyrhythms, articulation, pedals), awareness of the details of the sounds I produce, and awareness of my body, its choreography and sensations when playing. It's like a graph with multiple items, which in the smaller time scale fluctuate up and down, and in the larger time scale tend upwards. So I try to be aware of all of the aspects of playing and cognition of my playing and the music, where one thing might be frustrating me at the moment (like smoothness for a moto perpetuo at a brisk tempo) that can lead to imposter syndrome, but at the same time I'm learning about new choreography for the arms and hands that's really useful, and I also might have figured out how to better sculpt the dynamics for long continuous lines, which I'd never learned to do well before. Etc. The critical impetus for the imposter syndrome often has a modicum of merit (yes, I'd like to improve at X), but the emotional response often lacks perspective that learning is complex and occurs over many parameters (a lot of which are less flashy than the ones that frustrate us the most, but which are nonetheless essential to fluency), and that goals aren't necessarily always met instantaneously, and for complex things we often have talents for part of them and are not as developed as others (like for me, theory comes naturally, even advanced things beyond the piano like mathematics, acoustics and microtonal theory, while physically I have had to overcome a lot of tension just to do rather basic technical things consistently well). Learning is vectored and multivariabled, but sometimes we see it as static and scalar. When I feel imposter syndrome I try to determine what it is I'm self-conscious about, understand how that aspect is challenging to me at the moment, but also realizes the other things I'm doing and how that challenging aspect is a part of the overall holistic plan and is not the be-all and end-all of piano playing.