How do you know if you've got borderline personality disorder?

I have always understood and related to some of the extremes in BPD, such as the "all good/all bad" splitting, and the inability to see past my thought distortions. I also tend to either cling to people or become very distant, often because of tiny things they do that I exaggerate and blow out of proportion. I have huge mood swings, sometimes going between "the world is awesome!" to "I hate everyone and they all hate me" within a few hours. And I have a lot of the empty, alone, misunderstood, anxious, self-loathing feelings of BPD.

But I also don't have the really extreme symptoms. I don't threaten suicide, or have extremely risky behavior. I don't get into fights with people or get easily angered. And I don't regularly cut or self injure, even though I feel like I want to quite a lot. And although I internally react very drastically to relationships, it doesn't tend to actually affect them because I don't let people into those areas of my mind.

And if my T thought I was BPD, I'm not sure she would tell me.

I know a lot of these symptoms also fall under PTSD and my other diagnoses, too.