Dear {Grandparent},
I am sorry for the way I {behavior} when I saw you on {date}. I was angry because of {cause}, but that was no excuse for the way I treated you. I love you very much, and I would never want to do anything to hurt your feelings.
Next time we get together let's do {activity}. I would really like to try to make up for what I did. I have always loved doing {activity} with you, and I hope this activity will bring us closer together.
I love you very much, {term of endearment}, and I am very sorry.
Sincerely,
{Sender}