Fellow adult with ADHD here, welcome to the club! What you’re feeling is referred to as “imposter syndrome” and it’s one of the more pesky symptoms of our particular affliction.
Just knowing it’s a symptom of ADHD has been a huge help to me… When those thoughts creep in, I just remind myself how hard I’ve had to work to get here.
I know for a fact that I had to study for some of my licensing exams three or four times longer than some of my coworkers, for example. Those coworkers don’t know that, and I wouldn’t care if they did, but I know how hard I’ve had to work to get where I am and I’m proud of it.