How do I get over this painful memory?

My grandpa passed away when I was very little—about 4 or 5. I miss him so much. I wish I was a better granddaughter to him when I was little because I would always ignore him or think he is scary (due to diabetes, he had one leg). He was in the hospital numerous times and I did not pay attention to him. As a result, now, I want to turn back time and punch myself and beg him for forgiveness. I hate myself so, so much because of this, and cry myself to sleep every time I think about it. I disgust myself and I wish I knew better. How do I ever get to “regain” myself?