I'd Trade the Kingdom for Your Love Alone

[A normal beginning + an obsessive, unhinged hero later on + angsty, unrequited love]

To protect her and her family, the Empress Dowager forces her to disguise herself as a man and rule as emperor of the male-dominated Zhaoyang Kingdom. She must seize the Divine Maiden, secure the throne, and eliminate dissenters. But she never expects to be betrayed by someone, leading to the fall of her kingdom and the destruction of her family.

He is the ninth prince of Mobei, willing to do anything to achieve his goals. He takes the throne, destroys her kingdom, exposes her as a woman and crushes her hopes of restoring it, then forces her to stay by his side.