I'm young, and not even sure I have BPD, but in my experience, I have experienced symptoms much like BPD with or without a partner. I remember when I broke up with a bf that I dated for a long time and lived with that at first, I was like you. I hated being alone, felt an acute sense of abandonment, and I was afraid. But after that went away (never did completely tho) I could feel the relief of being without that person, tho I still had my issues, I think that the relief just was a new, and welcoming feeling that was much appreciated. I still struggle with my emotions tho, even when I'm alone.