What is the difference between narcissism and having accurate confidence in yourself?

Healthy & unhealthy: Having the capacity to feel good about yourself for the things that you can do well is a sign of maturity and healthy development. Narcissism per se can be either unhealthy (a person who is "full of" herself) or healthy (as you suggested, a person who has appropriate confidence in herself).

Narcissism: Narcissists will tell you they're the best at everything they undertake, bar none. Accurate self confidence implies feelings of being good enough at what one undertakes, but leaves space open for learning.