What have you done that you never told ANYONE?
My biggest secret was that I had regular sex with my high school teacher and his wife (also a teacher) from ages 16 to 19. They took me to orgies and hosted them at their house. He trusted me to be with his wife when he wasn't home, and, although she didn't approve, he and I sucked each other when she wasn't home or had gone to bed.
Obviously, I could never tell anyone! They got to know me and 'tested' me pretty well before they invited me, and of course were very strict with me about keeping my mouth shut. And I never told anyone until they were both retired (he died young), and I lived far away. My mom thought something was up, but she never imagined! She just thought that he would let me drink sometimes.
I have other secrets, and I'll tell them after you tell me yours...