I feel the urge to totally lash out at people in real life all the time, I've always either just tried to shut it down or at least be toxic in a non-violent way. Come to think of it I actually have done a lot of really insane things in my past, having extreme overreactions to absolutely non-threatening situations. I only developed self awareness in the last 3 years, before that I was not aware at all that I was overreacting, to me they were just normal reactions to awful situations. If that isn't textbook borderline I don't know what is.
So, when you were a teenager you didn't have the ability to regulate your emotional response.
Now that you've developed self awareness, you feel the feelings, but you understand that you need to shut them down - at least try and shut them down, somehow. And you are successful in re-directing them, often.
That's the textbook definition of not-meeting-PD criteria.
Remember, everyone has a personality, and sometimes, major aspects of those personalities are problems for the individual, or for the people around them. A personality disorder needs to demonstrate an enduring pattern, over time. That pattern needs to be markedly different from cultural norms and expectations. It needs to be pervasive and inflexible. Finally, it needs to lead to distress or impairment.
You barely have the 'over time', when we take into account the frame of having really poor impulse control as a teenage male. A few years after that, you developed what you call self-awareness (makes sense to me). It's
exactly that self-awareness that demonstrates you are attempting - and succeeding - at conforming to cultural norms. All by itself, that is a real big one, and is probably why you'd not really profile correctly for a personality disorder.
We can also add, re: BPD specifically, that your emotional dysregulation is not pervasive and inflexible; you have, in fact, gone to great lengths to describe how you regulate yourself.
You
are experiencing distress.
The qualities of a personality disorder - the pattern that is demonstrated, listed above: those qualities are all essential to a contemporary diagnosis of a personality disorder. You are likely only looking at the other area, where there are two of four criteria.
Self-diagnosis. Doesn't work. It's not a checklist. There are paradigms, fundamentals that lie underneath the criteria - and those are the things that take more years of study and/or experience.
Now, your therapist knows you. And they are (I'm assuming) qualified to diagnose (as in, have the proper amount of training, right license, experience, etc). So if your therapist tells you you have a PD, then they are right, and I am wrong. I might think your therapist is wrong, based on what you've said here.
Your assumption that you are textbook borderline is certainly wrong. You show more potential as an emerging narcissist, but only time would tell. Thing is, you've tempered your responses here with acknowledgements that you are reading and listening, even if you are disagreeing. So while your attitudes may be inflexible, your behavior still demonstrates some attempt to flex.
Just my observations.