You didn't do this and it wasn't your fault.
I know you don't believe that but that is the honest truth.
You were a child but also I don't even think that matters because at the end of the day he is your stepfather and and so that makes him wrong in every way.
You didn't instigate this and even if you had it wouldn't be your fault because he is the one who is supposed to be responsible and protect his stepchild.
I know this is so confusing and you feel like because you continued that you are somehow guilty but you are not this isn't on you.
I know you said he doesn't remember but if he doesn't that would be because he doesn't want to face what he did because he was so wrong,I tend to think he probably does remember though.
Are you seeing a counsellor at all? I would strongly suggest you do to help you deal with these confusing feelings.
Are you safe now?
Please try to keep strong and remember its not your fault.