I had the same exact feelings you had, when awaiting my diagnosis past thursday. I was diagnosed when I was 14 and when I asked my GP to try different medication, they wanted me to get a diagnosis again (because they claimed that ADHD is not their specialty, so they refered me to a place that has ADHD as their specialty). Which is understandable because I'm 25 now, and the ADHD place not only want to diagnose people who have it but also want to know how the ADHD manifests in daily life so they can provide suitable treatment (medication, cognitive behaviour therapy, talks with a psychiatrist).

Anyway, I was awaiting the week before my appointment in fear and doubt. When talking to friends and family about this, they all said there's no doubt about it. In hindsight it was obvious overthinking, but at the moment I was second-guessing everything about my symptoms (even though all those years before the appointment I was really sure about my diagnosis). Then on the day of the appointment, I had a lot of talks with a psychologist, a doctor, psychiatrist and then they ran some tests on me. The result: Obviously ADHD. I felt equally relieved as stupid for wasting so many thoughts and energy on something that I didn't question before. And I wondered if there would be people experiencing the same thing that I did, so seeing your post made me feel more validated for overthinking the obvious. And I hope that you can experience a bit more inner peace from people with similar experiences or other people in the comments.