I have similar experiences: I seem to say the wrong thing with some frequency and without being able to see why it was the wrong thing. It can be particularly problematic at work as I work in govt.
While I haven’t been able to solve that issue, besides asking people to check my responses before I send them, my psychiatrist recently gave me a great tool for when I am having obsessive thoughts about a situation. He said to intentionally have the obsessive thought — give yourself a cap like 1 minute or think the thought 5 times — then physically slam your hand down on the table or desk and say “STOP!” It really helps! It allows you to take back control of your thought patterns.
As for getting a diagnosis: there’s nothing to be afraid of. But I would definitely recommend seeing a psychiatrist or therapist who *specializes* in ADHD in order to get the most accurate diagnosis. Some doctors who aren’t familiar with adult ADHD may think your symptoms are something else (and they could be!) or, as in my experience as well as a friend’s, treat you like a druggie who just wants to get some pills. Someone who knows the nuances of ADHD that aren’t listed in the current DSM will be your best resource for diagnosis IMO.