Confidence, regardless of where one feels it, comes from an acceptance of one's self. This not only includes an understanding of what one is good at, but also what things one does not do well. I have found rock climbing to be a good way for me to build confidence because it incorporates a great amount of pushing one's body to the limits of endurance while also forcing the individual to trust that the person on the other end of the rope also knows what they are doing. It is much easier to be confident when you know can trust those around you. Do not try to build confidence, but build trust for if I know you will catch me when I fall, then I have no fear of falling. Confidence requires that a person accepts the possibility of failure, but fearlessly goes forward regardless. The largest roadblock to confidence is a fear that others will not accept your failure or that you will forever be judged by this failure. Once again I say build trust. Through trust, we gain an acceptance of each other's strengths and limitations.
Beware of anyone who tells you that there is a secret to confidence, that purely through selfish means you can gain confidence. This is a false dream that you can live in if you like, but it is fragile, waiting for someone outside of your dream to crush it, for you will still have no trust. If you cannot trust those you work with, find another job.