How can I gain more confidence at work and be taken seriously in meetings?

Here's the thing - I recently got a promotion at work, which I'm very pleased about but of course requires me to take on a lot more responsibilities. Im ok with the pressure except for one thing - having to do meetings with clients by myself or speaking up in meetings. I have always had social anxiety, but I don't think this is the only factor making me nervous. The thing is, I am fairly young - mid 20s, but I look younger than my age - I could easily p as a teenage girl if I dressed like one. Also I am the youngest of the family, and I'm used to people treating me like a child...even to this day. I guess I give off a sense of vulnerability or I look weak...I'm not sure what it is. So I'm petrified of having meetings with people in their 30s and 40s that are very strong and self-ured - Im worried theyll look at me and be like, wow she has no clue what she is doing or she seems too young for this. I am worrying myself to death about how I will do in meetings next week - any advice?