Yes I hear you, it breaks your heart seeing them like that and feeling helpless that there is nothing you can do. Everytime I saw her she'd either get a tiny bit better but gradually as weeks went on she deteriorated quickly as the cancer spread. I remember so vividly us all round her bed side just stroking her hair and hands as she was just unconscious (hours away from passing) at that point. It happens to the people we love most and we wish we could take it away from them but we can't and we lose our most treasured people. As I've said above though, all depends on what you believe but I don't think they completely leave us, if you take any comfort in that. I've had my signs to tell me so.
Please do not blame yourself, you did all you could by being there. I can relate I'm sure to how much you're hurting. Nans are some of the greatest beings we ever get the pleasure of having in our lives. I would say please take pride in the fact that she was a part of your life and your story and you had and loved her and will always love her. If you want to contact me privately if you need someone to speak to please please do I'd be so happy to talk to you. There's been some nights where I've done nothing but cry in disbelief that I can't see her physically anymore or felt guilty that I didn't see her more than I did.
Take care x