Jexa,
IMO, in any kind of abuse, the the physical violation is just the tip of the iceburg. I am not in any way minimizing it in anyway, because what your grandfather did was awful, in the most objective sense. But the emotional violation, the betrayal, is so impossible to comprehend. This was a person who was supposed to love you, care for you, protect you, cherish you, and instead he used you for his own se*ual gratification.
As others have said, the guilt is not yours. As others have pointed out, you would have felt guilty if you hadn't told, and this had happened to others. It truly IS a no-win situation.
I don't want to put the focus on myself, but just to let you know how I can relate, I often feel HORRIBLY guilty for telling on my abuser. It was a decision I had to make, it was the right thing to do, even though it came at a huge personal price. It sounds like you have also paid a terrible price.....It is not fair. NOT FAIR. There is no one who can tell you the decision and aftermath don't suck. But your grandfather is responsible for all of this.
About not trying to get him to stop - UGH! You were a child. He had no right to use you that way! He had ALL the power in the situation, and you had none. The fear and shock you must have felt......the confusion.....when someone is violent the fight/flight mechanism kicks in. But when someone is hurting us under the pretense of loving us, it is impossible to know what to do.
I wish I could say more, I wish I could 'convince' you of all of this. I struggle with the same type of thoughts, and I think part of the healing process is re-framing these types of thoughts, placing the guilt and shame where it truly belongs - on the abuser, NOT on the victim.