There's a difference between a secret and a sentence you're protecting yourself from. A secret is about someone else finding out. This is about you finding out — the moment the thought leaves your head and becomes a sound in the room, it stops being something you might be imagining and starts being something you know.
You don't have to say it to anyone. Just finish the sentence, once, even just in your own head: 'The thing I've never told anyone is—'