You were not only raped, but you were gang-raped and I would recommend counseling with a specialist to help you to deal with the crime that was perpetrated upon you.
I would also suggest that you forgive yourself - from your language in your post, you seem to feel that you should have "seen it coming" and not allowed yourself to get into the situation that you did. Nobody can predict such a violent, criminal assault! Something similar happened to me when I was 13 and it had a terrible affect upon me until I got involved in individual counseling
My best wishes to you, my dear. You were the victim of a CRIME and you are not responsible for the actions of others.