I wish...
somebody had taught me how to breath much earlier; how important the body is to playing; that the lips are the gateway to the trombone, but that the work is done much earlier; how important the mind is to playing; how important relaxation is to playing; that the instrument should be brought up to my posture, not the other way around.
These things I now realise and know intellectually, but they are not innate.
Does it matter? Well, I am where I am with orchestra, and I don't necessarily need to be at a higher level... But I do dream of it sometimes. We can all dream.