Ti789,
Everything you tell is very common in BPD. You are gonna find a lot of people here that share your pain. I'm really sorry you've been feeling so hurt, and so lost. We all have been there.
People with BPD have a base line (a normal state) that equals our "very stressed" state. Then when they are indeed stressed, they reach levels of "emotional arousal" out of the charts. That feels unbereable and they react in ways that are not conected to reality, to their goals, or their real feelings. Sometimes they are even aware of this, as if they were looking at themselves in a movie, unable to stop or to change themselves. And that brings them shame and a feeling of uselessness. Which in turn, makes things worse. Thinking about becoming a better person, or asking forgiveness, can stress them again and they chain crisis after crisis.
We need to learn about their individual experience, to understand what is happening. And then, we have to take change of our own actions, and our role in the relationship. Without the guilt of having done things that didn't work, and understanding our mistakes.
It's very common that people refuse therapy. She might change her mind. Give her time. Be patient and start reading the basic tools, you're gonna find many new ways to talk to her to start making things better for both of you.
One thing I've herad here, from the report of one person with BPD is that they need a target for their anger. That target is us, it doesn't mean that we caused that anger, or that the things they tell us are true, they can't just be angry , they have to be angry at us. But we are the ones that have to work out to understand it's not really about us. And learn to experience it in a different way. That defuses some situations. And the ones that it doesn't defuse, we have to learn to take an exit or set a boundary that they will learn to respect. Easier said than done, but there is room and way to improve things.
Stay strong.