I'm really sorry to hear that you're going through such a tough time in your relationships. It's important to remember that everyone's journey is unique, and what you're experiencing doesn't make you "broken" in any way. It's okay to have complex feelings and struggles.
It's great that you have a strong and supportive connection with your best friend in your QPR. Relationships can take many forms, and as you explore polyamory, it's crucial to have open and honest communication with all partners involved. Seeking professional help from therapists who specialize in BPD, c-PTSD, or relationship issues can also provide valuable insights and coping strategies.
Mental health is indeed crucial, and it's something I understand from my own experiences. When I suffer, I know there's mental health help near me.