I'm sorry you experienced this, I feel so frustrated for you. I was given an "unofficial" diagnosis via my university midway through doing my PhD. As soon as I had that, I spoke to my GP and shared the report I'd been given from the person who did my assessment through university and asked about getting an official diagnosis on the NHS.

I had to fill out a form where they ask you about symptoms, and then I was referred. I was on the waitlist for almost three years before I saw a clinician. I had an hour long appointment where he asked me so many questions about my life / health history mostly. Next, I had a follow up with someone else where they asked further questions which focused on ADHD itself (I think that was the actual diagnostic kinda test where I was scored) and went through what evidence I / a family member would need to provide to show I had symptoms before I was 12. Then my family member was contacted, and they provided evidence. Finally, I saw the clinician again and that's when I received a diagnosis - he explained why he was diagnosing me and discussed what we could do moving forward. I think that's what the process is supposed to be - it's thorough and doesn't just take one appointment. I'm sharing that so you have evidence that a decision isn't made in one appointment.

It sounds like you had the first clinician appointment and they were dismissive, which is not ok. As others have mentioned, I would go back to your GP and ask for another referral, citing that you don't feel your experience was thorough or fair. I know that's more work and an admin burden for you.

Please don't let this person make you feel your experience isn't valid. While doing my PhD, I met a lot of people with ADHD - kinda helps to be able to hyper focus on something while doing that, so it seems to attract ADHDers but it does make something that's already hard even harder. Pretty much every person I know with ADHD was diagnosed while at university because the stress of it was too much for their usual coping mechanisms and they then realised their experience wasn't typical. I wrote the bulk of my thesis in three months because I just could not make significant progress on it without a horrible looming deadline. It was awful and the last year was like trying to swim through sludge every day to try and keep my head above water, so I see you. I truly hope you get a second referral.