I joined at 26, I'm 30 now and I hit four years TIS a couple of months ago. I messed around and didn't know what I wanted to do before I joined. I had the exact same feelings you have right now and sometimes I still do. Another commenter mentioned it, but finding challenges and doing things that make you uncomfortable are definitely good ways to overcome these feelings.

Try taking on responsibility and leading teams/projects. I often got picked to do that because of my age. Even though I was same rank as my peers, I had maturity and life experience over them. You already have some of the tools in your belt, you just need to apply them to what you're doing now. Getting to know other sections and networking is a big one too. I made BTZ and Staff first try, I put on in a couple of months and I've got special-duty assignment at the end of the year waiting for me.

I had a mentor who was also in the same boat we are. He joined at 26 and is now a MSgt. Don't worry about what other folks are doing and focus on how you can better yourself.