How do I get doctors to take me seriously?

I live in England originally, but I go to university in Wales and am permanently registered at a Welsh GP. I'm 19, feminine-presenting but non-binary, I've got mental health and chronic pain issues and I have a history of 3 different antidepressants and the occasional painkiller prescription in my record. I'm also on the waiting list for an autism assessment and I have problems in social situations. I think that these aspects contribute to a prejudice doctors have towards people like me because I've seen this happen to similar people.

The majority of doctors, receptionists, pharmacists etc do not listen to me or take my concerns seriously, and I'm afraid it might be affecting my health. Recently I found out during an eye test that I have fluid building up behind my eyes which I need to go to an eye clinic for and I want to make sure that I get the treatment I need, because I've been subtly denied it for so many other problems. The only times that my problems have been taken seriously have been when I had a parent (or even a pharmacist from next door; long story) standing behind me. I don't want to have to rely on my parents for everything, especially when they live very far away from my university.

I need to know how to be taken seriously in medical settings, and hopefully help others to do the same. I think it's ridiculous that I have to use psychological tricks like tricking them into thinking I have authority in order to get treatment, but unfortunately it's what I need to do.