While the people around her including the second male lead (who lovers her) knows it, but they keep the male lead in the dark. Eventually the male lead finds out and is hurt that it was kept from him. He acts like he doesn't know the secret and hopes the female lead tells him, but she doesn't. He breaks it off with her and she tries to win him back as the second male lead pursues her.

Not quite what you're looking for, but close-ish:

  • Another Miss Oh | ML has a secret he's held from the FL. The 2ML sort of discovers that secret, but not in its entirety. Later on, FL also discovers the secret.
  •  Cunning Single Lady | FL has a secret she's kept from the ML, but 2ML doesn't know the secret either.
  • Moon Lovers: Scarlet Heart Ryeo | FL has a secret that she keeps from both the ML, the 2ML and the 3ML.