Confidence is an interesting beast. It took me a while to really see how it works.
Confidence is directly related with the Buddhist notion of "suchness" (and therefore with the so-called Emptiness).
To have confidence you have to accept everything at the most fundamental level, and keep accepting as things evolve. You have to fully accept the so-called external situation, and you have to fully accept yourself. Sounds simple but it's not.
When you accept yourself you commit to doing everything to the best of your ability but you also accept that you can make mistakes. When you accept the situation, and keep accepting it as it changes, you accept the outcome of your actions, whatever it is. You think about it, you do the best you can, you get a result, and you deal with that result. Rinse, repeat.
In other words, when you make a decision - any decision - either a microscopic level one or a grandiose one - you must take full ownership of that decision, you must stand on it with both your feet. Not be like "neither here nor there" "just in case" - no, it must be a fully owned decision. You commit to it, you implement it, and you own whatever comes from it and work with that.
Here's one more detail. To fully accept everything means: if you don't know something - you don't pretend you know it, you don't wish you knew it, and you don't try to hide it to save your ego. You accept that you don't fully know it, and when you talk to others you define the boundaries of what you know and what you don't. Then you work with what you do know, doing your best and accepting results, as described above.
Taken together this means that you stand firmly with both feet on the ground of the truth - the ungeneralizable suchness of things as they are, and you are being authentically and ungeneralizably yourself. Both externally and internally there is no lie or disconnect, there's truth, authenticity, and ungeneralizable suchness. You accept things as they are, you accept yourself, you accept your limits, you do your best and you accept the new situation that emerges without grasping at what it could have been. Whatever happens, you are not afraid - because you are not grasping at imaginary "could"s and "should"s, you work with what "is". You are true and real, every step of the way.
That's the power stance that gives you the real Buddhist confidence.
By now it should be clear that this type of confidence is an exact opposite of arrogance. Arrogance is based on "wearing an armor" - convincing oneself and others that you are perfect, consistent, that you know the only right way etc, defending your image and fighting for more. Buddhist confidence is based on "removing the armor" and being true.