Narcissism- what I believe to be the primary disease of our times-- is one side of a coin. The other side-- the narcissist's enabler-- is the borderline.
If the analogy for narcissism is "being the main character in their own movie," then the analogy for borderline is being an actress.
Her? Yes. Narcissists are mostly hes, and borderlines hers. (Not always, sure.)
The classic description includes: intense, unstable relationships; emotional lability; fear of abandonment. The borderline has no true sense of self.
Ironically, the borderline is a borderline only in relationship to other people. The borderline has a problem with identity only because other people in the world have stronger identities. Your Dad wants you to be one way, so you do it. Your boyfriend wants a different woman; so you do it. Your husband wants something else; so you do it. Who the hell are you, really? You have no idea, because you are always molding yourself based on the dominant personality in your life.
This si done mostly out of fear of abandonment: if you don't "be" the person they want, then they'll leave you, and then what? (Borderlines don't end relationships-- they end relationships for another relationship.)
The narcissist creates an identity, then tries to force everyone else to buy into it. The borderline waits to meet someone, and then constructs a personality suitable to that person.
If a borderline is dating a guy who loves the Dallas Cowboys, then for sure, she will love the Dallas Cowboys. If, however, she breaks up with him, and then dates a guy who loves the Giants, then she'll love the Giants. But here's what makes her a borderline: she will actually believe the Giants are better. She's not lying, and she's not doing it for him; she actually thinks she thinks it's true. Everyone else on the outside sees that it is obviously a function of whom she's dating, but she is sure she came up with it on her own. And she's not play acting: at that moment that she believes, with every fiber of her being, that the Giants are better.
Here's the ironic part: if a borderline was shipwrecked on a desert island with no one around, she'd develop a real identity, of her own, not a reaction to other people. Sorry, that's not the ironic part, this is: she'd become a narcissist.
The bordeline has external markings of identity: tattoos, changing hair colors, clothes. You may recall I said almost the same thing about the narcissist: the difference is, of course, the borderline changes her image as she changes her identity-- in other words, as she cahges the dominant personality in her life; but the narcissist crafts a look, an identity, which he then defends at all costs: "I would sooner eat fire ants than shave my mustache." Of course. Of course.
All those silly movies about a woman moving away, or to the big city, and she "finds herself:" that's a borderline becoming a narcissist.
If you look back on past long term relationships you've had, and are completely perplexed as to what on earth you ever saw in each of those people that kept you with them for a year; well, there you go.
This is why narcissists marry borderlines, and not other narcisstists. Two narcissists simply can't get along: who is the main character? Meanwhile, two borderlines can't be with each other-- who supplies the identity? The narcissist thrives with the borderline because she provides for him the validation that he is, in fact, the lead; the borderline thrives with the narcissist because he defines her. And, as she will tell you every single time, without fail: "you don't know him like I do." Everyone else judges his behavior; but the borderline is judging his version of himself that she has accepted.
Go back to my white high heel shoes example. The narcissist demands his woman wear white high heel pumps not because hem ay like them himself-- he might or might not-- but because he is the type of man that would be with the type of woman who wears white pumps. He thinks he's the sophisticated, masculine man of the 1980s, so she damn well better be Kim Bassinger from 9 1/2 Weeks. Blonde hair, white pumps. She could weight 400lbs, that's not the point (though it will become one later.) So she wears the shoes, and starts to believe she likes them, starts to believe that she is that woman. He reinforces this with certain behaviors or language towards her (he'll open the door for her, push her chair in, etc. You say, "well, what's wrong with that? Nothing, except that he ALSO beats her when she doesn't wear the shoes.)
It's almost battered-wife syndrome: what keeps her with tat maniac is that when he's not beating her, it seems like he is actually being kind to her, so great is the difference between being beaten and simply not being beaten. Meanwhile everything he does wrong has an external explanation: it was the alcohol, he's under stress, etc. And she's doing this rationalizing for herself, not for him, because it is vital to her own psychological survival that he actually be who he says he is, that he actually have a stable identity that things happen to, because her identity depends on his being a foundation.
That's why the therapist has to maintain such neutrality, consistency in the sessions. It's not just to avoid conflicts; by being the most dominant (read: consistent) personality, the borderline can begin to construct one for herself using the blueprints of yours as a guide.
If the borderline sounds like a 15 year old girl, that's because that's what she is. The difference, of course, is the actual 15 year old girl is supposed to be flaky, testing identities and philosophies and looks until she finally lands on the one that's "her." But if you're 30 and doing that, well...
--------(BTW, if you want to understand the mystery of women's addiction to shoes, here's my take: shoes are the article of clothing that represent possibility. Each shoe is a different look, a different character, and she can select "who" she wants to be that day. You might not notice the difference, but she feels it. This is not borderline-- it's normal, but it's normal because the shoe changes and the rest of her doesn't.)