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Hello and thank you for using JustAnswer. Given the situation and how you are still feeling guilty over this, I would recommend that you consider entering into therapy to help you move past this incident and free yourself from this guilt. Therapy is going to be solely for you and your own guilt and to help you overcome the sexual abuse that you alone suffered as well.
You need to start to realize that this incident with your cousin was a very long time ago and that it was an immature traumatic response to the abuse that you yourself suffered. You thought that this was "normal" given your own experience and that is because you never addressed your own sexual abuse in treatment. Your guilt is what is your belief that you need to be constantly punished for what you did, so you are holding on to this guilt as a way for repentance, but it is affecting you greatly. You need to look for a therapist that specializes in treating victims of childhood sexual abuse so that you can begin the process of healing as that is the only way you will be able to overcome this guilt and move forward with your life.