I had something similar happen to me - my boss said that I needed to sit up straight more, dress better, and show more confidence. It actually helped in a weird way, because it made me realize how I looked on the outside, and made me 'pretend' more - or at least try to - if that makes any sense... Pretending helps, because I find that if I can pretend long enough, I actually start to feel a little better... I wouldn't share more with your boss - he/she doesn't seem all that understanding.