How do I seem more confident?

I don't know. My problem is the opposite of yours so not really a problem to me. I'm an adult but feel like a 12 year-old child most of the time. Everyone I meet and any job interview I go for, people say I come across and confident and capable and actually inside I feel like a frightened child! Glad it doesn't come across that way to others but since you feel confident, I don't know how to advise you about showing it on the outside - sorry.