I agree and think this applies not only to playing the piano but to life. Even if you try to run away from, ignore, suppress or deny your feelings, they are still there. They will still affect you, and forcing it down (or into the dark as you say) often makes it worse.I think you need to work consciously on technique. Realizing that worrying about things, or stress in general can affect your technique is a part of that. But I made progress with those things by accepting this rather than fighting it.I hope I understood you correctly. Do you have some examples from your own experiences that could explain your point of view a bit deeper?
Tension is generally a bad thing both in singing and on the piano. Sometimes tension comes from mostly psychological sources, but sometimes not. My last teacher was very focused on psychological causes of tension, and she was helpful to some of her students who had had traumatic musical experiences or been shamed or browbeaten as young students. But often tension is just a natural response to a bad physical coordination. Like, if you slip on the ice and you make sudden movements to try to regain your balance - that's tension from reacting to a specific physical situation; you need better shoes, not psychotherapy, to get rid of that tension.An example from singing would be maybe you're trying to sing high at a moderate volume and you have poor airflow, the sound starts to fail, you feel something funny in your vocal mechanism and you tense up. You can fix that tension just by working on control of your airflow without fussing about the psychological roots of the tension. Or on the piano you may have tension in your hand because you are playing big, widely spaced chords without relaxing your hand while you move between them. I'm tuned into this because I had that very psych oriented teacher and it was frustrating to have her want to do psychotherapy on me when really I needed some specific mechanical advice to get around physical causes of tension. Once I stopped worrying about what the tension "meant" in a psychological sense and just focused on the mechanical causes, things got better quickly.
psychotherapy at a piano lesson? not that good. Is that even legal in your country?
I think you need a little bit of both, so that you at least do not rule out either or the other as you are working. I know examples both of people for whom a large part of the tension was psychological, who would have been help more quickly if they had been helped in working with that, and I know examples where the problem was mostly mechanical, and reworking what movements were used was mostly what was needed.Did you tell this teacher that you didn't need psychotherapy but concrete advice?