In my experience, even in normal relationships (what's normal I know, but I mean without the added stressor of addiction being involved) it's hard breaking up with someone. Its near impossible to be "just friends" with someone who holds a piece of your heart. I understand you care about her as a person and worry about her addiction but my advice would say, tell her how you feel about her and her addiction, how you wish she'd get some help, and then cut her loose.
When, or if, you break it off completely, don't consider it abandonment (I always got that guilt trip), sometimes you really can "love someone to death". You need to consider if you are enabling her--giving her money, covering for her behavior, paying her bills (household, legal, what have you), etc., basically doing things to "help" her. The "help" shows her no consequences.
I read somewhere how addicts are like vampires, they just Suck you dry until there's nothing left--everyone has a breaking point. I know that sounds terrible and I know addicts don't mean to do this intentionally, it's just part of their disease and how it affects people around them.
I'm new to this site and it's shown me a lot to consider with my recent ex boyfriend. I hope it can help you too.