Posture Affects Confidence in Your Thoughts:

Researchers have found that posture can affect not just how people perceive you, but how confident you are in your mind. 

A study involving college students had them in positions of good posture while seated or slumped over.  They were then asked to write about positive or negative characteristics of their future employment.

Surprisingly, the position they were asked to maintain correlated strongly with their thoughts of future job placement.  Basically, good posture gave them positive thoughts!

This is something that I have often thought in recent years.  After doing a lot of joint mobility training in the last six years, my posture has improved greatly, along with my movement efficiency - meaning, my everyday movements are more coordinated and smooth.

I think this carries over to how people on the street perceive you.  At 6' 5 (when I stand up straight!), I'm taller than most people and when you walk upright you seem to stand out more.  At least it seems that people take notice of me because of my good body mechanics and relaxed gait.

Which wasn't always the case, as I used to have pretty poor posture and ended up doing damage to my shoulders while lifting weights.  I think that stress and too much time seated can have an effect here, but your confidence levels and/or anxiety levels seem to also play a role.

At least, I believe it did in my case.  As a shy introvert with some social anxiety, I think my body positions reflected my confidence levels or lack there of.  So some specific training can definitely help and breathing exercises that emphasize abdominal breathing can't hurt either.

If nothing else, "fake it 'til you make it."  Meaning, if you stay positive and give off the illusion of good posture and confidence, maybe you just might make some internal changes to go along with the external.