I am so sorry for what you are going through..
I lost my sister to suicide. I remember one time when we all went to visit her in the mental hospital. Everyone else was so nervous, trying to pretend like there was hope that wasn't there, and generally trying to stop the equivalent of an out of control freight train from its inevitable demise. I (who was the closest to my sister), had a weird sense of calm because I realized both my powerlessness and her destiny at the same time. I was strangely in a better place than the rest of my family.
I am in no way saying there isn't hope for your daughter or comparing her to my sister, but maybe you are in the same place I was, in that you see the inevitability of your daughter bottoming out, and hard. You also know there is nothing you can do to stop it. None of this takes the sadness away, but it does take away some of the inner unrest and self doubt. It also helps us to not be as angry with the sick person, allowing us to focus on how much we love them.
I will pray for both of you.
HUGS