Have you told her what you've written in the second to last paragraph? Sounds to me you don't feel abandoned, not only. You're angry too. Angry at the sickness for making your grandfather ill and your mother so sad. Angry at your mother for not taking care of herself and perhaps of you? In a crisis, our child within can take over and it's not strange if you wanted the safety of your mother, even if you of course understand she had to help her parents.
Also, it sounds to me like you are hurting a great deal over the frustration and fear you've felt about your grandfather's quick passing. A year is such a short period to lose someone, especially to a disease like that, where you lose your loved one even sooner as their minds drift away. Maybe you feel he's the one who abandoned you and your mom? Maybe you wish you could've gotten more support from her since you were hurting too? It's not rational and you know in your mind she was already at her limit. Your heart nonetheless needed her.
Don't be so hard on yourself. Talk to your mother again and try to explain all these conflicting emotions.