In the end, for treatment, the diagnosis doesn't really matter. The symptoms do. But I do understand wanting a diagnosis and a correct one at that. A lot of BPD has to do with the past. For example: age of onset. This usually comes up in early teens. But you will not (or should not) get a diagnosis of BPD as a teen because normal teenage behavior can mimic it. Another aspect is trauma. Most have experienced some sort of trauma before the teen years. Also, there's your family. Usually, at least one parent has narcissistic personality disorder or BPD (or maybe something else?).

But it's hard to guess a diagnosis based iff of symptoms alone. BP and other personality disorders can have similar symptoms.