I've said it before as well, and will say it again...as the other kind folks have...you need to find different living arrangements, most especially for you.
Again, I understand where both of you are coming from. And she has hopelessly gone down the tubes...reached bottom...and she needs help. And YOU are not responsible for that.
What about your parents...and her parents? I would start there and let them know how difficult it is for you...that you, yourself, are still dealing with your own problems and recovery.
Did I tell you about my best friend...of 46 years? She has Borderline Personality Disorder. Her craziness is what attracted me in our teen years, I suppose. We became the best of friends. But in the past decade there were so many issues with her that dragged me down...down...down...and down! And then, just a couple of years ago, something happened with her that was too insulting for me to deal with. Especially in the depressive state I struggle with from day to day.
I couldn't take it and started ignoring her phone calls. Which wasn't difficult to do. People with BPD are extremely difficult to deal with and always feel like everyone else is responsible for every single teeny tiny thing that happens to them. And even though SHE was responsible for the final blow (among many other blows) she never, ever thought that she WAS the responsible person. It was always my or someone's else's fault that she was having her exaggerated problems.
Anyway, my best friend. I haven't seen her since that horrible explosion she had. It was just too much. One of the hardest things I've done. And I do miss her...we have so much history. BUT I don't miss the daily tribulations with her.
In other words, sweetie, you need to do what is best for you. The confines of your living arrangement are too difficult and most likely won't result in the best outcome for either of you.