She thinks she's always been good at keeping secrets.
Her mother and the King loved to spy on her, and it wasn't easy to sneak out and meet Maleficent, but she did it anyway. She has lived in a secret for twenty-eight years. Even now, she looks at Emma and has to be quiet about Snow's past sins.
But Maleficent knows her – if she senses that she's hiding something, there won't be forgiveness.
Even after all their past caresses and unshed tears and affection, Mal always looks at her as if she's about to burn her alive or kiss her.