Which way to let go....

My mother and I really started our relationship (She was never around) when I became an adult and my children were little. Fast forward years later and my daughter is going through her teen rebellious days and moves out to live at Grandma’s house (my mother’s home) which I was against. She allowed my daughter to do whatever she wanted to, which is why I didn’t want her there. After everything settled down, and my daughter turned 18, we let everything go. I learned that my mother did meth with my daughter. I don’t know how to deal with that. My mother is hooked and my daughter is fighting with it. I really don’t want anything to do with her. I haven’t seen her but I’ve helped her out several time since finding this out. I’m fighting with what to do. I don’t think I can ever look at her again. I think about other things and feel like the bad person, for example: What about when she’s old? What about when her husband dies? My children have a relationship with her.