You feel guilty for leaving someone who
- Often didn't come home due to indulging in her addiction
- Didn't think your concern for her warranted calling you to tell you where she was
- Didn't reciprocate your affection in the earliest months of the relationship ??
You said it yourself...being with her was like a mission...like perhaps "saving" someone?
You cannot change her or cure her. Only she can do that for herself, and it sounds like she's not interested at the moment.
Sounds like you did just the right thing.