its really not your fault.
for starters, shes a grown women. if she choose to start because you suggested it or presented it...it was still her choice.
life is a series of those, you have some hard ones.
1, continue sinking together because you dont care.
2, continue sinking because you feel guilty and cant separate.
3, get away. hit up rehab, move on.
you may FEEL guilty but again, you did not force her to get into it and you deff didnt force her to leave a rehab because "shes not that bad" or whatever bullshit SHE used to convince herself.
you cant help someone who doesnt want to help themselves.