My grandmother and I were extremely close but unfortunately she had Alzheimer's and Dementia for 5 years. And this year I'm going to prom May 18th and my grandmother about 1 week before she died purchased my dress for me. I just found out yesterday too. But I cried when I found out, and I miss her so much. And you weren't selfish. I bet if your grandmother were alive she would say go to homecoming, because that's what she wanted you to do. So don't feel bad on yourself.