How to be taken more seriously/find an attitude

I am 22yo and look like a 17-18yo and I feel like I’m not taken seriously wherever I go. Be it in a formal situation such as going to the bank for whatever reason or paying bills, people always look down on me

Besides I have a lot of trouble expressing myself around others

I was just wondering wether or not you ladies know of ways to fight this or how to lose fear of rejection and other people’s’ opinions.

Thanks