This is one of those posts which I'd like to return to every once in a while, and read it again and again 
I discovered it just today and somehow I tend to feel sorry for not having been able to comment on it yet (because I was really busy).
Thank you, Wolfi, that's very nice of you

. And you, too, Goldy

.
I know that your soul comes out in piano just as when you sing. Whether in piano or voice, the soul can probably be blocked in each case for psychological reasons, as you mentioned, such as not having confidence in oneself. My voice was affected by that. I think singing and playing offer a different experience of the same soul.
It may be true that my soul can be heard, I can understand why it would seem that way. But, for some reason with singing I haven't felt exactly shy about that in the same ways I have with piano. On occasion it's been the case and I can pinpoint it to a couple of specific circumstances, but there's almost always been something more revealing-seeming to me with piano. As though it is something more meaningful or deeper for me, and as though I have to actually actively hide myself to an extent. Recently, I have not hidden myself while I play and it's been some kind of huge, life-sized relief of my soul, especially the first time I did that. But sometimes I wonder if I just haven't found the one particular teacher yet with voice who brings more out of me than I am willing to admit on my own, and more than the teachers I've had have ever brought out. With singing, it most of the time feels more like it's just sound vs. my soul, and that's why I've felt I can hide behind it, I suppose. Most of the work I've ever done with teachers in voice has never even come close to the kind of spiritual or inner experiences I have with piano, and sometimes in my piano lessons. Except recently, in my last singing concert and the rehearsals leading up to it, I had a breakthrough along those lines because in a moment I related it to a certain thing as I think in piano playing, and suddenly I was more connected from my soul. It's not been as scary for me though as with piano playing, regardless.
The confidence issue is an interesting one. Sometimes for me, "confidence" is mostly just a matter of grasping the difference between various experiences that I'm already having, or grasping more fully what the expectations are and how "the game" works. Once I know how to do something, I don't feel I actually lack confidence in my ability to do it, but if I'm unclear on expectations or how to fulfill them, that can be problematic for me. I suspect it's actually similar for most people. Once a singer, for example, starts to recognize the difference between their natural sound and an affected one, then they start to be able to reproduce it and as they do that, they seem to naturally grow in confidence. As a teacher, a big part of my job is pointing out when there are in fact differences in sound and which ones we want vs. the ones we don't need. Some people catch on right away to the differences and others take longer for one reason or another.
I agree that singing and playing are different experiences of the same soul, but there's something in the very middle of the their individual expressions which they somehow seem to share. I don't think that thing in the middle is just my actual soul, it's like it's a kind of active mushy spot with a bunch of options of avenue, but the outer crust, the actual manifestation is in fact different or experienced as something different by one's outer ears and perception and by most other witnesses.