For me, it's just a bit of cheerleading that helped.
Even though I'm currently feeling "stuck" in my grief and depression.... there was a time in my therapy journey when my confidence was better.

Just gaining my T's trust, learning to be comfortable with him, and getting to know the ways he would react to things, helped me to be that way. One of my confidence things with him was becoming assertive. I'm typically not with people but it came quite easily with him, in fact, he pointed it out because I wasn't even aware of it.

One of the things we worked on was trying to build my confidence so I could get back into dog training teaching and he was my "student"... it was hard and awkward at first but it got easier. We would make it funny and joke around and he would take a genuine interest in it and ask me a lot of things and give good feedback.

I truly believed at that point, he believed in me, and that made all the difference in being able to do those things. I suppose for some the support and cheerleader type thing can be helpful

The other thing I've constantly heard and read online is fake it until you make it. Never tried it myself but it might be worth it. Also I've heard mindfulness can help with this, maybe someone here who has done mindfulness could weigh in?