If you suddenly love a movie or band because someone you like expresses their admiration for them.... And you're not sure if this person just tuned you into something great that you were missing out on, or if you love this new thing just because the other person does.

I think it manifests most in ways like this, that your preferences or passions keep changing all the time and are conflicting. If your values change like one month you hate drug abusers and think they're worthless but then you meet a guy you like who happens to use drugs and your values completely change. Or if you're a Christian and then meet a new best friend who is an atheist and you stop going to church. Those would be signs of a lack of a sense of self.

In most people, their identity is fairly stable over time once they've passed adolescence. In those of us with BPD, it can shift suddenly for reasons we don't quite understand and we might not even notice the shifts or see them as contradictory.