Oh my god, I had no idea this was a symptom of BPD. I've never been diagnosed with it, but I have some of the symptoms.
I've been infatuated with the same person for almost 9 years... And I'm not talking about a romantic partner. I thought I was alone in my craziness lol. I've been obsessed with different people (some romantic, some not) since I was young. But this particular infatuation is almost a part of my identity now. 9 years, and I'm only 25, so a big part of my conscious life. Luckily, this person lives halfway across the world and we can never be in contact again. When we had contact, I acted crazy and desperate around him, and it hurt us both.
No therapy, medication, or approach has helped me with the anxiety I feel about human relationships, and I've decided to give up on them altogether. My infatuation is actually my security blanket, because I can love this person from far away and he will never hurt me. He's like a God to me, always silent, but omnipresent. I will never find anyone who can compare with him, so I've stopped looking. I'm completely isolated from people, and as hard as it is, it is actually better than when I was seeking love and attention from them. I want nothing, expect nothing, and it's been so long since I had any kind of intimacy or touch that I don't even miss it.
I wish I could give some advice on dealing with this, but I can't. I guess I'm just sharing my experience in the hopes that someone can relate. My only advice would be to stop interacting with the person you're infatuated with. Do whatever it takes, ask them to block your number if you have to. That's the only way, the closer you are to them, the more it will hurt you both. Accept that this is not a healthy dynamic and you can't be together until you learn to manage your emotions and stop being infatuated with them.