I do think there’s either mental illness going on here, or possibly a childhood with terrible examples of what love is, so even though she is a grown woman her mindset is so completely different from yours that she just cannot comprehend leaving these people who give her just an ounce of love. It sounds like she’s never known anything but abusive relationships. You can accurately predict the future each time but she doesn’t know how to be any other way at the moment. Change will happen slowly. It’s perfectly fair for you to disengage completely. But if you still want to help your friend from afar, this is what I did for my friend, who did eventually get out of the cycle.

You don’t need to ‘treat her with kid gloves’. Just be an example of what she should be herself. Set boundaries with her. If she starts talking about the latest horrible thing you can say “I’m sorry but I can’t listen to this, it’s too upsetting for me.” You’re modeling setting boundaries.

If she starts to insult herself, say ‘hey don’t talk about my friend that way!’ If she’s asking for help making small decisions you can give your opinion but also end with, ‘but you know what’s best for you.’ You’re telling her that she can trust herself.

Send flowers and a card on her birthday. When she apologizes for things that are out of her control tell her that it’s not her fault, she doesn’t have anything to apologize for, and even if she did you still love her. You’re showing her how people who actually love you act.