I want a diagnoses and too be taken seriously, but I am high masking and high functioning, how should I go about this?

When I was 16 I was diagnosed with anxiety, depression and ocd. Which I believe to be true although the person who made the diagnoses was extremely unprofessional. Since then I've had a really hard time with trusting professionals. I think I may have add along with autism and I've done a lot of research and found out a lot of adult women are dismissed when it comes to a diagnosis. Is there anything g I should do before seeking a diagnosis, anything I should know or prepare for? Any advice would be appreciated.