Oof. BPD and drug addiction is a one-two punch, and treatment is not easy even if she was motivated to get help. To go through cancer treatment without her support must have been terribly painful.
What's also tough is that she sort of has things the way she wants, so there's not much to motivate her to seek help or get better. When you try to make a change, she has a way to stop you from moving forward, so that strategy is working for her.
seems enamored with the idea that she has an actual mental health diagnosis and uses that as a get out of jail free card. "That was very hurtful...." "I have BPD! It's incurable! There is nothing they can do for it!"
How do you respond when she says that?
Many of us stay in these relationships because we're afraid of (insert worse thing) and the fear, obligation, and guilt (FOG) works like a jail cell to prevent us from leaving.
Do you have a therapist in your life? The confusion and FOG of trying to leave someone who is suicidal is really tough, especially the combination of fragile victim and abusive bully rolled into an intimate partner.
It sounds like you feel it isn't financially feasible to live independently, and you fear for her safety and well-being if you were to leave?