@virgo31, i think most of us have felt this way before. I was in a similar situation - I moved up rapidly through the corporate ranks when I was in my 20s. By the age of 31, due to a unique shift in the regulatory landscape, I found myself being hunted for roles that are normally reserved for someone at least 3-4 years my senior. It was terrifying to show up first day of work and have those fears and anxieties that you may actually be an imposter. Relax, it happens to all of us.
There were some initial bumps and I hated going to work everyday. I hated to be called the "subject matter expert" and I hated it when people started asking me for approvals, to "confirm this" or to "approve that". At one point, even the little junior who worked under me seems to know the subject better.
However, the more I worked, the longer I stayed, my reputation grew quickly. And I began to realise that I was indeed the subject matter expert I thought I wasn't. I worked hard, but a lot of it came down to my existing knowledge and experiences in my previous jobs. That was the reason why i was hired!!! My junior, who initially seemed to be more knowledgeable than me, really was because he knew the system inside out. Once I familiarised myself with the system, I quickly became the de facto SME because I had the technical training he didn't.
After a few more months in the job it quickly became apparent that even my own boss suffers from this syndrome. While she may put on a "know-it-all" face, she was actually just as panicky as I am in those high level meetings we attended together.
Everyone suffers from this. So don't feel bad about it. That's really all i can say - hope it helped, even if just a little bit.