I have also received and accepted an appointment to USNA and I had the exact same feelings. I had been working towards USNA for years but when I actually received my appointment (after the initial burst of excitement), I had a big feeling of "Oh crap...what am I getting myself into?" I took 5 days before I made my decision and accepted. What ended up helping me get to that point was talking about it, asking current/former midshipmen all of the questions I could possibly think of, and taking the pressure off of myself to make a rapid decision. Whether I decided to accept or reject the appointment, my future life would not be the same going forward. Either way, my childhood experiences are coming to an end and I am moving into a new chapter. A lot of my family wanted to know what I was thinking about everything but it was important to remember to separate their opinions from my own. I think it will help you to try and do some of the same and think back to why you started this process in the first place. Maybe reread your application and nomination essays and find your 'why' again.
If it makes you feel any better, I still have waves of fear and uncertainty. But, after all, "Regret for the things we did can be tempered by time; it is regret for the things we did not do that is inconsolable." - Sydney J. Harris
I don't know you, but I think you will always regret it if you don't at least try attending the Naval Academy. You can always leave during the first two years with no payback if you hate it.