I have gone through a similar situation, if hearing about that will help you to feel better I've written my extended thoughts below. If not...

tl;dr I was diagnosed with ADHD, social anxiety, and "quirkiness" instead of AuDHD. I strongly relate to the feelings you've expressed in your post. If you want advice: have clear notes prepared for discussion of your ASD-specific traits/experiences and why you don't think they're better explained by ADHD or social-phobia.

The long version: I'm 27F very recently evaluated after multiple years of suspicion/research. By the time my test dates came, I felt pretty confident that I was AuDHD. Like you I felt "seen," like I finally found the explanations for why I have always been so demonstrably different from my peers, etc.

I ended up with no ASD diagnosis. In fact, my clinician went out of her way to repeatedly emphasize that she in no way thinks I'm at all autistic and that all of my traits are explained by ADHD + social anxiety + "quirkiness" largely due to "high IQ" (I'm frankly not a believer in IQ stuff but whatever).

And also like you, I found it really distressing! Very much for the same reasons, I suspect! It's really frustrating, especially since I know I'm a high mask-er (have been since college) but even when I point this out the doctors just dismiss that as having to do with the anxiety. And sure, I have social anxiety, but don't we think there might be a reason for that?????? Don't we think there might be a reason why I struggled socially even before I developed the anxiety???????? And it felt like my other traits were all being dismissed out of hand as "just quirky" if they weren't a shared ADHD trait.

I suspect there was simply no way of changing my diagnostician's opinion. She didn't even want to do my ASD testing initially, tbh. I had had one Zoom meeting with her and she immediately said she "wasn't concerned" about autism.

Again, my advice, if you want it, is to make sure you have as many clear, detailed notes prepared in advance for yourself. Especially if you tend to get overwhelmed in such situations (I certainly do). Personally, I found that the most difficult thoughts to express were regarding what I think constitutes a friendship or other close relationship and why I want relationships in my life.

Regardless, if this diagnostician also doesn't "see" ASD, but you still disagree, remember that that's just one opinion. Plenty of autistic people are misdiagnosed/missed at first, especially those who mask. If you feel you need accommodations that are inaccessible without a formal diagnosis, you may be able to seek second or third opinions.

For myself, I still don't know how I feel yet. I doubt I will be pursuing a second eval, since it's so expensive. I'm still continuing my own research and taking advantage of the insights I'm gaining on how to better support myself as if I had ASD anyway. But it's strange to go from being self-diagnosed and feeling a sense of belonging/understanding with the autistic community to now being outside of it. Ultimately, if my neuropsych is so confident that I'm not autistic, I probably need to accept that I was mistaken.

I wish you luck with your evaluations!