Hello,
I am sorry that you are going through this, especially since your granddaughter was on such a good track for so long.
Your GDD is already on the road to making decisions that will affect her for the rest of your life -- what she needs is a bottom line. I would suggest to stage an intervention. Right now, she has "permission" to do what she likes, because, at the end of the day, she has a safe home to return to, and therefore she really is not feeling the full consequences of her behavior.
So, stage an intervention, and during it present a contract -- listing explicitly what rules need to be followed in order to live in your home. Also, demand that she enter therapy, and have a place set for her to go. If she fails to oblige, set that bottom line -- give her 90 days to move out. If she agrees and then goes on the break the rules or drop out of therapy, or does not take therapy seriously, give her 90 days to move out. She isno longer a child, really -- she is not 11 or 12. She is 19. Remember, there are other 19-year-olds that are fully supporting themselves right now, or working their way through collenge. She really is capable of handling herself as an adult, if she chooses, and, obviously, legally, she is an adult. As heartbreaking that this may sound, you are not responsible for saving her from herself -- only she can do that, and you can only support.