I would always recommend people get a second opinion from an ADHD&Autism specialist, because unfortunately many medical professionals go through all of their education/training without ever actually being taught any thing about ADHD. Many doctors’ knowledge of mental disorders are also decades out of date.
A decent chunk of doctors just know ADHD (and many other disabilities) off of stereotypes and/or misinformation, it’s a common thing in the medical field sadly.
Many times people are either misdiagnosed (sometimes a person with ADHD might also have another disorder which causes the doctor to not think it’s ADHD causing specific problems) with an anxiety disorder, BPD, a depressive disorder, or even Bipolar, unsurprisingly ASD too. Symptoms often present similarly to those.
Many ADHD symptoms present similarly to a depression and/or BPD, and ASD & ADHD symptoms can overlap a good bit (but they’re not the same despite some people thinking ADHD is part of the ASD spectrum).
Presence of symptoms also causes anxiety (as in when you were in school and you had an assignment due in a few hours, but you can’t force yourself to do it, but you don’t want to do anything else because then you’d feel bad about doing that instead of the assignment, so you sit for hours in a pool of anxiousness and ADHD paralysis while not doing the assignment even though there’s theoretically enough time to)
ADHD also has extremely high comorbidity and link with other disabilities, like the ones named above as well as things like Schizophrenia, Dyspraxia, Dyscalcula/Dyslexia/Dysgraphia.
The biggest problem though is that there’s this massive stigma against ADHD, it’s seen as a bad thing for someone to have ADHD (especially to parents of kids with ADHD, who often deny it vehemently and take it as a personal attack) and there’s this perception of it not being real or serious, plus there’s a whole myth of it being universally overdiagnosed (ADHD is actually extremely undiagnosed, despite some people getting the opposite impression that doctors just say every kid has ADHD these days). It’s just something conflated with this concept of “laziness” or “just normal things everyone experiences”, like a lot of other disorders are (victims of chronic depression and panic attacks can often relate)
Basically always get another opinion if you’re still not sure. Someone with ADHD that only finds out later in life may have to change their entire view & approach on the world/life and learn to come to terms with how current society works against them, and how they can try to minimize the massive disadvantage they’re given in most situations. So knowing is pretty important.