As with many goals that we struggle to attain, we tend to go after them in a way that blocks our success. Quite often I find that individuals who quest for more self-esteem - more internal security and confidence - actually get in their own way. Typically, they are prone to measuring and judging themselves against others.
Such individuals attribute more confidence to others and arrange to always fall short in their comparisons. They see their vulnerability as a weakness and try to mask it. That is the source of the problem. We are all vulnerable and insecure in one way or another. When we accept that vulnerability and no longer seek to hide it, we are actually strong!
The person who tries to act strong or confident may well be masking their concerns and doing so serves to exacerbate the problem. We do this when we are more concerned by what we think someone else thinks of us than coming into acceptance with our own self.
There's a paradox here. Embracing your vulnerability and no longer hiding from it means that you're no longer concerned about others' judgment of you. When you act in such a manner your relationship with yourself surges. This is strength and confidence!