There is a huge issue with what has been taught as "posture" and it has messed up a lot of us. People are starting to wake up to it - things are popping up all over the place: piano, any musical instrument, yoga, and even trainers who have hulks doing chin-ups. The stuff that messed us up is still being taught.
Two ideas - If you can afford it, find a good Alexander Technique instructor. (I couldn't, so I haven't.) Another is the book "What Every Pianist Should Know About the Human Body" by Thomas Mark. A third: if you can get a hold of a good teacher who is body-aware, technique-aware, and can observe you with open eyes and without memorized rules of "good posture".
I got the book after having already worked out a few things, and everything there seems true. He tells you how your body really works and has you explore. The book does not work if you only read it, or if you select bits and pieces here and there that you think might suit you. It's something to work through section by section. You'll probably discover that half the battles you have are caused by misperceptions about the body, and that many of those stem from what you have been taught about posture in the first place.
There is also Mary Bond, and then there is an odd seeming character who actually makes tons of sense who has a site called "The Smiling Back". He got into Alexander when he was a piano student and now he teaches it.
Personally, I am working with a teacher who does not follow any "posture rules" but is able to observe what looks effective and what doesn't. So far everything he has said has gone hand in hand with the book by Mark - I draw on what I learned there to help me reach some of what my teacher says, and in turn, my teacher gives feedback on what he sees and hears from me. I give my teacher feedback on whether I feel comfortable for what he suggests. So far this is working.