You mentioned your father twice. You also mention connecting with this girl at a time when you lost your father. I also notice you call your father, "dad," a verbal anchor which demonstrates a strong emotional imprint. So you likely feel a "hole" from your dad's loss, and here come the BPD to the rescue to fill this hole. And you the "savior" could not deny help to another who needs it, especially to one who is servicing your "hole" unconsciously.
I understand what it is to lose family. I lost my entire immediate family at a young and older age. I am here alone. I will die alone. It's quite empowering to need no one for your physical, mental, and emotional well being. It's further empowering also to at one time have the world against you, l-i-t-e-r-a-l-l-y lose everything including your entire wealth, friends, and family--but nonetheless slowly climb out inch by inch in the darkness--and then succeed in a big way. No tidewater melancholy, no yearning for lost causes, just purposeful and determined direction and action. My life has shown me that nothing is impossible. Everything you desire--no matter, who you are, your upbringing, your education or lack of, your "limitations"--can be achieved.
Your father was great. But he served his purpose (among many others), which was to help make you into who you are today. Now take those lessons he tried to instill and put them to work--in his honor. Would he have allowed his son to self-deprecate with a BPD? Your life can be anything you want it to be simply by the choices you make and intransigent execution of the game plan. Choose wisely friend.