Re: Think that my partner may have Borderline Perosonality Disorder
As reading your detailed description, your boyfriend may have Bipolar that is very similar to BPD his cousin has. As you know, mental ill runs in the family.
Besides this fact, the anti-depressant brings on other mental ill, particularly Bipolar, if he predisposes it. In Bipolar, there are two sides as the word, 'Bi...' imply. One is sunny side,... nice and genteel while the other is dark-side,.... angry and combatative. In between, there is also the normalcy to get back to himself/herself once awhile. It's one of annoying and complicated mental illnesses, to say the least. For that reason, most of psychiatrists advise his/her patients to not disclose his/her mental issue unless absolute necessary to do so.
Your quote, 'self-medicated' gotten from his mother is not really a good idea, because there are different reactions to different anti-depressants.
Try to pursuade him to go to the psychiatrist when he's in good mood. As much, you're available to visit to the doc. with him and tell his current condition to his doc. as detail as possible. Because, there are no other good tools for the doc. to dx. and prescribe med. appropriately.