take a/the secret to (one's)/the grave
To not reveal a secret for the duration of one's life. I can't believe that grandma took such a huge family secret to her grave! If Uncle Joe hadn't blurted it out, we'd never have known about it! Oh, Emily is a loyal friend—she would definitely take the secret to the grave.
take the (or your etc.) secret to the grave
die without revealing a secret.