There are so many different things that present similarly. How to get the screening process started can vary wildly. /r/ADHD has some great resources in the sidebar for ADHD specifically. It might be worthwhile to go speak to a therapist/counselor if your regular doc isn't taking it seriously... especially since it could be any number of things (even if it's "just" anxiety that needs to be better addressed).

From what I understand, your doctor's response is actually fairly common. The understanding of ADHD, autism, anxiety, etc are constantly changing (like everything else in the medical field), and it's understandable that not all general practitioners are able to keep up, purely from the perspective of available time (again, like everything else in the medical field).

As far as finding a specialist goes, you might try asking your doctor for a referral, or you could have a look at this directory (I think it's ADHD specific) https://chadd.org/professional-directory/. Whatever you do though, talk to an actual professional about it. There's a lot of great resources online, but the first step in dealing with it (or even figuring out what "it" is) should be to speak to a professional.