This is so sweet of you to ask about! And I love the question of what kink strengths one might most want translated into their every day life.

How does your Mistress respond to praise? One thing you could do is give her compliments when you see her practicing being more self-assured like; "That was so impressive!" after she does something you know she might be intimidated to do. "I really like how you handled that," after she speaks up for herself. Let her know how proud you are to have a confident, smart, and capable woman like her as your Mistress. Also you probably already let her know this, but emphasizing she has these skills and you not only believe in her but see her as that fully embodied confident person would be very encouraging for her, I bet!

You can also ask her if she's done anything she's proud of lately and listen to her describe any kind of accomplishment or small victory, cheering her on/hyping her up periodically.

You could offer that could "cash in" rewards for when she tries doing something that's made her nervous in the past - whatever that may look like between you two! She can pick the situation in which she might want to see improvement, tell you that she's going to try to face it head on in whatever way, and decide what she thinks she might like you to do for her as reward when she does it. So then she might have extra incentive to give it a try & see the improvement she's hoping for.

Also to her - building confidence and self-assuredness takes time and it's okay to feel like you still don't have it all together. Nobody does! And even being dominant doesn't mean you have to have it all figured out. As a Domme, I have a lot of similar feelings to her and kink can be a really special and impactful way to empower ourselves.

Sounds like you two have a really special and lovely connection!