You can leave, and should. She is in trouble for sure but likely much stronger than you think. Remember too that what people do generally has to do with themselves, not you.
The question to ask yourself, maybe is "What do I get from this relationship?" because not wanting to leave means there is something in it for you despite the toxicity---and it isn't likely to be a healthy 'need' or desire. It is really important that you be able to live with yourself before forming an intimate relationship. It is OK to be alone, and more likely you will be able to meet your own needs. Once you can meet your own needs, you can think of meeting another's. In the meantime there are casual/activity oriented Meetup groups (or go for volunteering somewhere) that can help address both loneliness and the need to engage with something outside yourself.
It isn't easy advice, I know that.....((((((((((hugs)))))))))))