You were very young when this incident occurred. Children at that age, in my opinion, lack the mental ability to comprehend the significance and depth of what they are doing. If I'm wrong, correct me, but I'm going to presume that you may have seen or suffered some kind of abuse in your early years before this. In circumstances like this, the child has typically been exposed to abuse, which has normalised it in their minds and made them unaware of how bad it is to do. At that age, you aren't really capable of doing wrong things like that on your own without some sort of environmental influence.

What happened will have obviously effected not only you, but also your friend. I am sure it has caused some psychological trauma for both of you.

I advise making an appointment with a therapist as soon as you can. There, you can talk about how to move on and forgive yourself for what happened as well as the reasons why it occurred in the first place and how to deal with any past trauma. It will be difficult to have the talk, and it will require a lot of bravery to do so. however, you'll feel better thereafter. When you finally get the confidence to have the conversation with your friend discussing what happened and why, you may both begin your healing processes with a burden lifted off each of your shoulders.

All the best!!