Self-esteem and confidence are built and maintained through our own recognition and acknowledgement of our successes. While some people seem to have the edge from birth and are naturally more confident than others, positive self-concept (as psychologists call it), is not a fixed attribute and it can be developed at any time.
Neither is is everlasting; attributes like self-confidence and -esteem need maintenance and renewal if they are to remain intact and serve us well (this goes for the confident people too).
Life has a way of eroding confidence by reminding us of our failures, imperfections and shortcomings. The simplest and most natural antidote is to count your successes, recognise your strengths, and acknowledge your resources such as experience, achievements and support (by family and friends for example).
Think about how self-confidence works for a moment; when you have it you are much better able to resist being swept up with thoughts that tell you you are rubbish (or are blame-worthy, should feel guilty, undeserving, or whatever). But we all have chinks in this self-protective belief, and so occasionally doubts can creep in. When that happens it distorts your thinking and confidence is threatened.
Consciously focusing on your competencies and other affirming attributes can help counter this effect and so, boost your confidence, self-esteem and resilience.