I think most autistic people who reach adulthood undiagnosed are forced to self-diagnose, either as a first step toward getting a professional diagnosis or because a professional diagnosis is impossible to obtain.
For every one adult who says that a doctor or other professional suggested that they might be autistic, nine others will tell you how hard it was to even get a professional (GP, psychologist, psychiatrist, social worker, etc.) to consider the possibility, let alone recommend that they get evaluated.
As adults we basically have to diagnose ourselves and then go to a doctor armed to the teeth with evidence to get them to see what is so obvious to us.
This is actually a sad truth Other people have long ago decided we can’t possibly know what’s wrong with ourselves....
This, and do yourself...favor and educate yourself on conditions