I am not sure if this will help you, but I can tell you I know exactly how you feel. Because I had huge imposter syndrome for the first couple of years as a flair here. It wasn't until the mods and other flairs told me that my research process is the same as many others, and what I thought were sub-quality answers I wrote were definitely up to standard and better than 90% of the crap they remove did I put that feeling behind me. By they way I don't have a MA, and I'm right now preparing for my MA entrance exam which I may very well fail. So you are already better than I am. You may think you're too young and inexperienced, but all I see is someone who's bright and energetic to such an extent to achieve so much at such a young age.

It seems like you already have a goal and a rough idea on how to get there. As long as you remain motivated to keep going you'll be fine.

I would say pick an area in history you already have a passion for and make some noise about it. I was in a similar situation when I went from a small Southern college to a nationally ranked graduate school. Like you, I felt intimidated at first - but on the other hand I knew my Southern ancestry back to the colonial days and I had several patriot ancestors who fought in the Revolution. My knowledge and advocacy for colonial Southern history made an impression on people and I eventually felt like I had earned a place at the table.