After a gloomy week in which I've felt dissatisfied with my playing, I've found this thread both interesting and encouraging.
In particular, Coalport's comments have "struck a chord" with me! I think I've been aware of Zen-like aspects of theremin playing but never really put it into words. Apart from a comfortable tai-chi-like stance and calm breathing, I know that the less I "think" about the playing the better it tends to be - and the more I "think" while playing the worse I get. When I say I tend not to "think" I DON'T mean I'm not concentrating; I AM. I tend to be concentrating so completely that I'm totally absorbed in the music, and any conscious, calculating thought is superfluous ... or even distracting. If ever I find myself calculating where the next note should be, (or looking at my hands) I invariably miss it. I tend to "zone-out" and stare vacantly at something ... anything ... a chair leg or a few square inches of wall (NEVER anything interesting) without seeing it or focussing on it. And then play with my mind empty.
The more relaxed (and empty-headed) I feel about playing the better I seem to play; playing best of all when it's just purely for the fun of it and there's no one listening. If anyone IS listening, or even if I'm just recording my practice session, thoughts wander in and that total absorbtion wanders out! (Recording practice sessions is something I frequently do; equal parts helpful and depressing when listened to later!)
I totally agree that fear of failure can become a self-fulfilling prophecy - but I've always found that, even years ago when I played the piano in public; if there was ever a passage that I struggled with, I tended to worry about it as it drew nearer, and such distraction invariably lead to me playing as badly as I feared I would.
With the theremin, once in that kind of semi-hypnotic "altered state" I find I don't have to think about the next note or whatever finger-shift is needed ... it just happens. For that reason, I even find that reading sheet music will break the concentration, so I like to feel I really know a piece well before trying to play it (if that makes any sense). At first, that zoning-out" happened as an accidental by-product of intense concentration, but with practice I've found it possible to ease myself into this state-of-mind as I start to play ... and judging from some of the trance-like expressions some thereminists have, I suspect I'm not alone.
However, it's still easy to let anxious distracting thoughts slip through ... and this thread has reminded me that THIS has probably been one of the causes of my dissatisfaction with my playing lately.
My other problem: I think I tend to forget just how insubstantial the playing arc is; no matter how carefully I tune and position myself it IS annoyingly variable ... but, on days when it seems to be trying to evade me, I feel like I'm failing somehow and get discouraged. I also agree with the comment above (Jeff S) that over a period of time learning to play the theremin, one's pitch discrimination gradually improves. The discouraging thing about this is that, as your likelihood of hitting right notes steadily improves you become ever increasingly (and painfully) more aware of the notes you miss ... which might explain why I often feel that I'm getting worse rather than better.
Which is why I especially appreciated the reminder that playing the theremin is "a delicate musical tightrope walk". I suspect I have to get the right balance between striving hard to improve and not being too hard on myself when I don't!