First and foremost, thank you so much for having the courage to share your story. Your words touched me, and I’m sure everyone reading this post, and you never know when you can truly help change another life just by sharing your experience. Secondly, none of this is your fault, not in the least, not one bit. You were a child, plain and simple. It was not your fault for ‘going along with it’ or ‘encouraging’ it, it is not your fault not telling your parents, it is not your fault, and nothing you did caused this to happen. You were a child, and one that was taken advantage of from someone in a position of trust and who was supposed to be looking after and supporting you. Just leave the thought behind that there might have been something you could have done differently. The only thing you are responsible for are the things going forward and your decisions now that you are an adult.
I do agree with some of the other comments that it often helps to talk about things. Thing kept in the dark can stay festering inside of us, burning us from the inside if not. Just the fact that you felt the need to write something and share it, indicates you feel a need to be heard. You should tell your parents. You might not get the reaction you expect, either way, and you have to be prepared for that. But, at least the not knowing and the desire to share with them — to explain — will be fulfilled. How they react is on them, not on you. That is their responsibility. It might help to talk to a friend, counselor, random person online, as you continue to deal with this.
Above all, be kind to yourself. No one should ever have to go through what you went through. You are a warrior to still be here.
All the best, and sending love and good wishes for your continued journey.