Going through your life well into adulthood while being told "Oh, that's normal. Everyone does that." can make it a bit different. People in this situation may also learn to try to compensate for it, but it can sometimes only do so much; until they hit a brick wall and can't further progress with their lives.

The constant questioning to yourself of do you actually have the disorder, maybe you just need even more sleep, or maybe your mind is just making it up can plague adults who've dealt with it unmitigated their whole lives. The feeling does fade, but it can take time.

I imagine the culture which a lot of millennials also grew up in, with D.A.R.E. and the rebounding effect of the war on drugs could also play a subconscious factor of thinking "meth" is bad and only done by meth-heads and crazy people.

If you need ask that question, I'm not sure I could offer an answer that may be satisfactory, or relatable to your own experiences (because it most definitely differs)! Though, I know it can come with its own cons for some, it's fantastic that were able to receive a childhood diagnosis, and don't feel similarly to adults who grew up with their ADHD symptoms being dismissed to the point that we regularly question our own validity. I have my own [insert sob story here] reasons for why I doubt myself, but no, I don't doubt my diagnosis.