i hate to break it to you: it is the person you fell in love with.
if you choose the path of loving someone with borderline personality disorder, it is a hard truth that will confront you again and again, until you accept it.
only then can you make the chances necessary to get your relationship on a better, healthier trajectory.
if you are going to pursue this relationship, do so with eyes wide open. know that while things can improve, its a difficult path.
this is a great place to start:
https://www.bpdfamily.com/content/what-does-it-take-be-relationship