Having a sense of self is knowing who you are. People with BPD usually have an underdeveloped sense of self. In my experience they are usually raised by a parent or parents with a cluster B personality disorder, usually narcissistic or antisocial. Both of these disorders manifest a near total selfishness and lack empathy. The world revolves around them and they give nothing to those around them, including their children.
In a normal, healthy family the parents nurture the child and help them explore the world and develop their own sense of identity and self. The parents respect and support the child’s development of an identity separate from theirs. For instance, Mommy is coloring with her daughter. Mommy’s favorite color is red, but she notices her daughter likes purple. Mommy notices this, mentions it, and celebrates it with her daughter. “Oh! You like purple. How pretty that is.” Mommy not only allows her daughter to be different from her, but she reinforces that her daughter’s choice of a different color is just as valid as Mommy’s and that is very pretty. This encourages the child to develop their own sense of self.
By contrast, when a parent is narcissistic or antisocial their child is merely an inanimate object which exists to meet the parent’s needs. The child is not allowed to have needs of their own. If Mommy likes red, everyone must like red. If Daddy likes basketball, everyone must like basketball. If Mommy is hungry, everyone is hungry. If Daddy is tired, everyone should go to bed. Their child’s attempts to be individuals are thwarted and/or attacked. When coloring, narcissistic Mommy will deride a child who chooses purple. “Why are you coloring that purple? How awful! You should color it red. I hate purple.” Notice the implication that, because Mommy hates purple, the child should too. This child, instead of developing their own sense of self, learns very early to morph into whatever Mommy wants. They learn to meet other people’s needs. They learn not to have needs of their own. They grow up not knowing what their needs are, not allowing space for their needs, and not knowing how to meet their needs.
As an adult, this child will morph into whatever their partner, friend, or boss wants them to be. They have no internal compass, no sense of who they are, no sense of what they need. Narcissistic Mommy has groomed them to be what others need them to be, and they are very good at doing that. This is why relationship breakups are so excruciatingly painful - and threatening - to someone with BPD. They only understand themselves as a reflection of someone else. When that person leaves, there is nothing to define the person with BPD. They are left with this gaping void in the center of their self which is terrifying.
A large part of therapy with someone with BPD is unlearning the indoctrination of a narcissistic/antisocial parent and helping them develop that sense of self.