Dan Jones has written a beautiful poem called "Shameless" which has a few lines which I think are priceless. He is talking about the nature of being shameless as well as what self-confidence is, and isn't. It's absolutely beautiful...
Vanity and conceit have nothing to do with this,
and not arrogance or righteousness.
Those are the masks of Shame.
The "proud" man, the "vain" woman,
the smug, the selfish, the egotistical and stuck-up,
all the "better-than,"
the know-it-alls and judges and busybodies,
inside they squirm with unworthiness worms
and have no love for themselves.
Self-love is good humored and easygoing.
A self-loved body tingles with pleasure
like a sun-warmed beach
when waves of love wash over.
A self-loved mind is at peace,
does not turn against itself in criticism,
does not strive to make up for itself with a good show,
has no fear, no anger, no greed,
no hurry, no worry,
just glides along like a white cloud
doing one thing at a time
as though it were serious.
This says so much and says it so well. Self confidence is not boastful or arrogant. It is quiet and accepting. It is not righteous or judgmental, it is compassionate. When we accept ourselves completely, with all our frailties and our foibles, we more easily accept the shortcomings of others. If we are judgmental, highly critical and intolerant of ourselves, we are equally hard on others.
We also tend to project our shortcomings onto others in an attempt to deny it in ourselves. Our shadows are powerful. That which we try to repress threatens to eat us alive. You can see this with homophobia. Who are the most homophobic people? Those who suspect they themselves may be homosexual and try to deny it. Their intolerance of their own sexual orientation creates what Freud called a "reaction formation" to homosexuality in general. It reminds me of Shakepeare's Hamlet who says, "The lady doth protest too much, methinks".
Another reaction to shame is to be more sensitive to the same flaw in others. Alcoholics Anonymous has a saying, "If you can name it, you have to claim it". In other words, flaws you quickly see in others are probably the same things you tend to do yourself. Things which annoy you most when others do them are probably things you hate in yourself. A client who complaining of talking non-stop and not being able to shut herself up once complained that a coworker talked incessantly and never shut up. She felt she was going to go crazy if she didn't get a transfer away from working next to this colleague.
You can read the entire poem at the "How to Live Happily" site. You can also read more about Self Confidence and Self Improvement on this site.