Hi, I'd like to help you with your problem.
It sounds as if you friend has chosen her life with drugs and addicted friends over her friendship with you. Often times, when someone becomes addicted, they adopt the lifestyle that goes with the addiction. It is hard to maintain a drug addiction when everyone around you is sober. As an addict, you want people around you who are doing the same thing as you, can support you in your destructive habits, and who approve of your lifestyle.
Along with addiction often comes a shedding of old habits from before the addiction. This includes relationships such as family and friends. It sounds as if your friend has embraced the addiction and therefore has moved away from you as a friend.
You have done what you could for her and have been there for the friendship. There is little more that you can do to maintain it. She is no longer doing her part and when only one person is keeping the relationship going, it cannot work.
It is probably best you move on. You can tell her nicely by saying that you feel you both have moved in different directions and no longer have anything in common- or something similar to that. I believe that if you chose to continue with the relationship, you will probably continue to be hurt by her and possibly by her friends.
Since you have been friends with her such a long time, you might experience some mourning over the lost relationship. That is normal. You may feel sad, regretful, angry, or disappointed. Let yourself mourn but if you feel it goes on too long, seek the help of a counselor. Short term therapy may help you overcome the feelings and be able to move on for good.
I hope this has helped you,