Well, I can see ur a good kid. I can relate because I went throught the same thing when I was your age. It's the Jesus Complex. You want to help her, be her rock, be the one she can count on, and hopefully she will see the light and stop what she's doing and clean up. It's difficult. She has to WANT to change. I don't know how strong her will is, but from my experience, she will have to hit rock fkn bottom, near death basically or selling herself to support her habit. Then that will be the turning point. Hopefully it doesn't get to that point, but most likely it does. Sometimes ppl never change. It's a bad situation and it's alot of weight for you to carry on your back for 19. It could prevent you from going places. Just keep on moving forward and like it or not, you will have to move away from ppl with drug addictions as you mature. Your "group" of friends will dissapate and you will meet new ppl as you move on in life and better yourself. Don't get caught up in that bullshit "I don't ditch my friends bro" attitude. When your an adult, that shit doesn't matter. I'm 26 and the friends I left behind are still doing Ketamine and Coke. They've become losers IMO. They accomplished nothing except doing drugs and being in the perpetual mindset of a 17 yr old.
GL and take care of yourself.