A guilty secret
Definition 1 of 1
Pro Tip 1/3Negative Emotion
The phrase often indicates feelings of shame or remorse about the secret. 
His embezzlement became a guilty secret that haunted him.
Pro Tip 2/3Disclosure Fear
It suggests fear of negative consequences if the secret is revealed. 
She couldn't face her family with the burden of her guilty secret.
Pro Tip 3/3Personal Conflict
The phrase could involve a personal moral conflict, indicating an inner struggle. 
The stolen money was a guilty secret, tearing him apart.