The problem I'd have if I did decide to leave is that it would feel like I would be giving up on her that and the results of the break up could make her situation even worse.
You may feel like you are the only thing standing between her and a total meltdown, and maybe you are--but you are not necessarily doing her a favor in preventing that meltdown. As long as she is protected from the results of her drinking, why would she stop?
As long as you cover for her, clean up her messes, put up w/broken promises and lack of responsibility, nothing will change--at least not for the better.