Autism, ADHD, Depression, Anxiety, and Private Healthcare

Hello, everyone. I was diagnosed with ASD in 2018, and while that has helped answer some questions, I'm still struggling to piece other things together. The truth of it is, while none of my family are diagnosed for it (as far as I'm aware), a lot of my family members display symptoms of pretty debilitating inattentive type ADHD, as do I. I am a pretty sickly person in a more general sense, but to be honest I don't go and speak to doctors as much as I should. I'd like to be tested for coeliac disease too, and have multiple other issues. I'm just super disheartened by the whole process, because whenever I go to speak to anyone (which isn't often, I haven't seen anyone since 2019), doctors are always incredibly dismissive of my needs as an autistic person, but also about the issues I bring up with them.

So I guess my question is this: How do I broach the idea that I think I may also have ADHD comorbid with the ASD? Many doctors will probably still believe in the old diagnostic criteria that we can only have one or the other, and I'm scared of going there and being dismissed on that basis.

Another question I have is that of the potential to cut to the chase and be privately assessed. I have never gone the private route for anything before, so I have no idea how to get into that and I'm scared of being scammed. Does anyone know of any reputable services that provide ADHD and other mental health assessments, so that I could potentially bypass the whole process of practically begging for referrals from a GP? What I'd really like is a full, deep dive sort of psychological assessment. I can't imagine having anything like that going through the NHS, and last time I tried it went incredibly poorly.

Last time I saw a psychologist for a more general mental health assessment, I masked but was honest about everything. Except, because I masked, I guess the psychologist misread me. I told her I was having frequent suicidal "fantasies" about some of the worst and most dangerous ways a person could harm themselves, but I also said I would never act on them because I wouldn't want to hurt my family in that way. I don't have these level of thoughts anymore though, so don't worry about that. It's largely been replaced with (pretty severe, imo) anhedonia and depersonalisation. Anyway, because I was so straight to the point about all of my thoughts and feelings, and was masking, I guess I seemed more put together than I actually am. She diagnosed me with mild anxiety and depression, and sent me away with no further support. If I wanted to speak to someone again, I'd have to speak to my GP and they'd have to put me on another waiting list.

So I'm honestly scared of going back to a GP and asking for anything at this point. So much stuff has happened, more than what I've outlined here. I feel very disillusioned by the whole process. All I want is to start feeling better, to have my person and situation respected, and to actually have help to get better and achieve my health goals.

Sorry if any of this is unclear, since this message was written quickly on the spur of the moment. If you have any questions regarding my situation, feel free to ask and I'll do my best to answer. Thanks in advance for reading all of this, and for answering. It's greatly appreciated.