I'm sorry, too, kate. I never had to deal with addiction in a child (yet--I sometimes think my younger son is "in training"). As hard as it is, you are doing the right thing. Your "helping" her the way she WANTS to be "helped" will only prolong her suffering. There are many places she can get REAL help if she is willing--and usually that kind of willingness requires a certain level of desperation.
Hugs, stick around here--you'll find lots of support and love.