I've read through all of this, and I can tell you this:
Whether she's jumped back on the drug bandwagon or not, she's got her head messed up. Basically, she's turning 18 soon. You're turning 21 soon. I doubt that she's figured out what living on her own is like. She'll find out soon. You, on the other hand, seem to have been out there for a while. You did what you could with this relationship. I'd get on with my life, if I were you. Stop checking up on her. If you have any reason to chat with her folks or friends, don't ever bring her up in conversation, again. Doing so will only keep you wondering WTF?!
I, basically, killed a relationship by constantly wondering about my ex. That's why she's your ex, because it's over. I spent YEARS trying to work things out with a girl, to no avail. It was on, then off, then again, then off again.
Your idea of living life is a great one. Just remember, don't be a dick, when/if she comes crawling back to you. Because if it was a drug problem or not, you weren't a dick to her before. She's still a human being. And, you'd hate it if you were treated the same way. Turning her away would be just as good as what you'd say to her. Just say, "No, Thanks."