Are people born with borderline pd? is it hereditary? or is it developed?

Yes and No, or Both: Every baby is born with some temperament, some personality. Part of everybody's mental state is inherited. The rest of it develops with age (the development is guided by both inherited and environmental factors). Nobody is born with a blank mental state, that is capable of developing into all types of personalities. Some aspects of every person are already predistined, but nobody knows how much.

Object constancy : We all internalize healthy image of our parents as children so we can grow up to have healthy adult relationships the process of crystallizing these images in our brain creates healthy self which is not possible when there is abuse trauma absent or sick parent leading to void or disturbed object constancy so these adults grow up with constant instability of mood dependency of relationships hard to.