Confidence is not something that we can teach you. It’s more-so something that acts as a bonus feature. You will be hard pressed to find a black belt who has no confidence and that’s because they had to push themselves to get there. They put in the work, the practiced, and they finally achieved a goal they set for themselves.
Then on top of that they have the knowledge that they can take care of themself if ever presented with a scary situation. There’s no way that person lacks confidence. When people spend time working on themselves then that can easily become a mentality.
There is a lot of clichéd statements out there about being confident and so I don’t want to add to that. However, I will say that confidence is one of the most useful tools that self-defense can give you.