I’m sure this is very, very painful for you. Perhaps I can offer a possible explanation that will help you be patient. Your granddaughter is trying to understand who she is. She has reconnected with the mother who abandoned her. Teens in this situation often feel that they have to become like their parent in order to not be abandoned again. In addition, it is the fantasy of most teenagers to be free to do what they want when they want. It isn’t healthy for them. It isn’t a good way to live. But they often have to find that out for themselves. Your granddaughter is a 17-year-old who is desperately trying to make sure her mother won’t leave her again and who mistakes neglect for freedom.
Please trust that all the years of loving her and providing for her and teaching her will eventually come to her rescue. You didn’t raise a stupid person. She’s just confused about how to be loyal to both you and to her mom. Avoid arguments with her about what she is doing. You can’t win the arguments and you might lose the girl. Just let her know that you love her and whenever she is ready you’ll be happy to have her come back. You could tell her that you fully understand why it’s important to join her mother’s life for awhile. It does make sense. But her mother’s life hasn’t been good for her mother so it may not be all that good for her either. If you say that, be sure to say it without a critical tone. Reassure her that she’ll always have your love and that you hope she’ll visit. Then just love her, love her, love her.
I know it’s hard. I understand that you are scared for her and sad about the rift in your relationship. Please have faith that you raised her well and that she will eventually come back to you. It’s more than likely that she will.
I wish you well.