Hi 8888an8888, please DO NOT take the guilt that should be rightfully his. I'm sure you've heard this before..........but there's no not saying it............it was not your fault!!!!
You didn't "let him", he did it...........there's a massive difference there.
You were in no position to consent, you didn't have the power to stop him, and that's completely understandable........you were a kid. It was something that happened to you and you would never, ever have chosen it, right??!!
And I am sorry that it happened to you and that no-one spotted the signs
But you know, massive respect for speaking out about what happened to you, that alone says that you have amazing strength!!
So please keep on getting the help you need, I know it will be a far from easy journey working through what happened to you, but with that strength............!!! You do not need to be all that he did to you........and with time, with help.........
And your grandma...........**** what she believes, we know better, right??!!

Alison