Is this my fault? what can I do to get over it?

It is NOT your fault. But you definitely need to talk to someone about this. Something like this will stay with you forever and you need to talk to a professional to find a way to cope. I'm so sorry that this happened to you and you don't have to blame yourself for this. You were young and truly believed that they would kill you and your family if you told anyone. Please talk to someone, even a parent or a friend you trust. Tell them about how you feel guilty and let it all out. It won't change the past bottling up all those emotions aren't good for you.