Imposter syndrome. Quite a few people in my position have this - the feeling that someone's one day going to realize that I'm not really a grownup and say "What are you doing here? You're just a little kid! Go home to your parents!"

The strange thing is the grownups in my life either don't remember this or have never experienced it. I describe this feeling and ask how long it takes for it to go away, and they're all "What are you talking about?"