The only way to to overcome fear of working with people who talks louder and are more authoritative is to focus on facts and do your homework before meeting them. No guarantees of coming out ahead, but if you aim for win-win, however it turns out, you will come out of it almost without an injured ego (since you're focusing on facts) but most certainly you will come out of it learning something.
Therefore, face your fears and be fearless, with facts and due diligence on your work. Know your limits and acknowledge your weaknesses, only then you can work on moving forward and pushing your limits...being fearless.
