Yes and no. You are firmly stuck on a double edge sword. Yes, because no one has the right to depend so much that "you can't take it anymore." Self-preservation has to be number 1 unless it hurts someone else.
The no comes in, because we all have obligations to help each other. But ask yourself if what you have been doing has helped. It doesn't sound like it, so we are back to yes you did the right thing or at least something different than wasting your time and hers.
Lots of double talk, huh. Double edged swords cause that. My sister and your cousin have a lot in common. I finally had to make a choice like yours. Years passed and the whole time, I worried that she would die (she couldn't get any worse).
Finally, one day she started doing better and she's still improving. I've kept the shield up to protect myself, but we talk more and I am happy for every little improvement.
The strangest thing is that she doesn't blame any of us for tiring of her escapades and only slightly for rejecting her when she was so needy. I'd be a lot harder on me, if in her shoes.
The answer to your other question about why she sticks to something that doesn't work is fear of change.