As hard as it is, especially with a child, even a grown child, it may be time to think about letting go of being involved in her issues, her responses - not feeling obligated to either reprimand or defend her. She is an adult, and this is hers to solve. It may be more powerful for her to go through this without filtering her choices through what you think of what she's chosen. It is possible that that reinforces some dependence in her behavior.
I think it's a great idea to write a letter with no holds barred about exactly how you feel. You might want to do it as a journal, and add new entries and revise and change as your feelings change. As to what you send to your daughter at rehab, you might only want to write a few sentences.
This is so tough, and my thoughts and prayers are with you. Remember, we'd didn't cause this, we can't control it, and we can't cure it. Your daughter is young, and there is great hope for her if and when she takes full responsibility for the totality of her own behavior and her own life and future.
Here's hoping she'll wake up and take on the challenge!
ShootingStar1