I have a lot of experience - several years professionally - with mental illness and drug addiction. Twice (that I can recall), I have made potential diagnoses in my answers regarding people the OP is having problems with.
I was asked about the wisdom of doing this by a moderator, so I stopped, even though I have pretty fair confidence in my ability to diagnose given enough information (and I would not diagnose if that information was not presented in the question, either deliberately or unintentionally.)
One thing I would not do, however, is to try to diagnose the OP. For one thing, it's unprofessional. Would you want a bunch of people on the site to read that you were probably a narcissist? Even though it's only an SE [gr]avatar? Probably not. Secondly, it's tricky. It's easier to diagnose someone that a person is talking about (when they have no reason to misrepresent facts) than it is to diagnose the person who's talking (who is probably presenting themselves in a selected light).
Yet on several occasions, I've seen this done, and have flagged a moderator about it, but nothing was done, most recently, on this question: How to deal with anxiety/control issues in relationship.
In this answer, the user suggests that the OP may have Borderline Personality Disorder (I doubt he means Bipolar disorder.)
My flag was deemed helpful, but the answer remains, unchanged. (The underlying cause of BPD is an intense fear of abandonment, but there's a lot more to it. Lots of people have abandonment issues but don't have BPD.)
What do we as a community want? Do we diagnose or not? (N.B. When I diagnose, I cite references. This should be the minimum requirement.) I'm ok with not diagnosing, but not with the inconsistency.
Edited to add: I would have edited it, but, in case no one has noticed, I also answered that question, which would make it entirely inappropriate for me to edit. So the diagnosis has remained (as has the double standard.)