Ouhi-sama wa Rikon Shitai: Isekai kara Seijo-sama ga Kita no de, Mou Oyakugomen desu wa ne?

The Kingdom of Lienrhein welcomes a person with memories of another world as a saint and queen. According to this rule, Elys, who has the memory of a "rekijo" (a girl who likes history), becomes the queen, but she is married for political reasons and is forced to live a life where she does not feel the love of King Leenhardt. When Haruna, who has been teleported from another world, appears, Leenhardt's feelings for Haruna begin to grow. One day, when she finds out that the king has spent the night with Haruna, Elys finally makes up her mind, "I'm leaving this place!"

(Source: Kodansha, translated)