Years ago at UC Berkeley we hosted an "almost famous" pianist in concert. We had two very fine Steinway concert grand pianos, but the artist was unhappy with both, so we had to rent one from a shop in San Francisco. A couple of years later we invited Richard Goode, one of the finest virtuoso pianists of our time. He arrived at the venue early, and when I came backstage I heard music from deep in the bowels of the theatre where the stage door guard had sent him. I went down and found him playing an old beat-up and vaguely out of tune upright rehearsal piano. I asked him politely if he wanted to rehearse on one of the Steinway pianos. He said to me, "I'm not doing this for the piano, I'm doing this for my hands."
Therein lies the difference between confidence and self-esteem. The almost famous artist must have the "perfect" piano in order to perform, whereas Mr. Goode can make an old beat up piano sing.
When you present yourself to a potential employer, are you propping up your confidence by talking only about your accomplishments, credentials, and experience, or are you communicating who you actually are? You may be confident in your abilities, but you must be able to convey a sense of who you are and what your values are. Remember that your experience and abilities are only part of what an employer is looking for. Mostly they are looking for someone they genuinely want to work with. For someone with a low self-esteem, being able to communicate your authentic self is risky. Nevertheless, you have to try. If you are shy, it's OK to be shy. Just speak your truth. How the potential employer responds will tell you everything you need to know about whether you want to work with them as well.