To me, being self-confident is feeling comfortable with who I really am. Not someone else's idea or opinion of it.
That is something I have always struggled with, caring a little too much what others think of me. Naturally I am rather shy but I am also very friendly. I love people but I don't like being the center of attention or having the spotlight on me in any way. Someone told me once that I lacked confidence - they were very wrong!
My self-assurance comes from knowing who I am on the inside. There is a very fine line between confidence and arrogance and I would always rather err on the side of caution. Every day I learn a little more about myself and as I do it is easier to let some of that confidence shine.