My partner is struggling with is what suspected to be BPD, or at least BPD tendencies. (Actively in treatment). 

Disclaimer: I am not at all a mental health professional. This is based off of my experiences with this person, their family history, what *my* therapist suspects based off of my experiences/their behavior and my own research. I am not assuming a diagnosis, and I’m aware that I’m incapable of coming to any for sure conclusion - I’m simply here in hopes of gaining more education &amp; tools to support my partner. They meet the DSM-5 criteria for BPD. 

Ultimately, they shuffle between demonizing me (and everyone else in their life) and treating me like I’m the best person they’ve ever met. Within days, they switch between “I’ve never felt so loved by someone as I do by you”, to “all you do is hurt me” with no apparent reason 

We’ve split up in the past, and I’ve only ever offered to work on our issues however we can. They’ve switched between, “I don’t want to be with you”, to calling me their partner within days with no explanation/communication. I asked them where we stood, and they flipped on me again in less than 24hrs. That night, we had plans with a friend the next morning and within a day, they blocked said friend and told me we aren’t together because I asked if we were together. 

I’m just confused, I love them more than anything and just want what’s best for them. If they’re not willing/ready to work on our relationship, that’s fine. If they want to, I’m here for it. I understand this is all related to their history of mental illness, so ultimately I just want to know how best I can support them during these times. I’ve been giving them space to process their emotions. 

TLDR - How can I best support/handle someone who’s suspected to be splitting? 

Any advice, input or support is appreciated. TIA.