You really have done the right thing. How lucky your Grandson is having you looking out for him, it doesn't seem that his mother can at the moment.
A relative of mine had the same issue with her granddaughter. Her own son (the girl's father) died before she was born and the mother went off the rails with drugs. The little girl was lucky to have a Granny like you, who fought for her through the courts. She now has full custody of her granddaughter and she is coming on well.
You have done all you can for your daughter, time to concentrate on your grandson.
Good luck. xx