THE CURSED PRINCESS AND THE SUN PRINCE: I WILL MELT YOUR FROZEN HEART

"I will never accept you. Never." As a mysterious illness and relentless monsters push the sunless kingdom of Noimont to the brink of ruin, Princess Hermine-known as the "Cursed Princess"-resolves to save her homeland through a political marriage with the grand and prosperous kingdom of Grantia. Her intended groom, Prince Dietrich, is known as the Sun Prince for his brilliant mind and striking beauty. But their engagement comes with one condition: Hermine must open his heart within one year. Cold and distant, Dietrich rejects her from the very start, and Hermine quickly realizes just how difficult that task will be. But with warmth and unwavering determination, she sets out to melt the frozen heart of the lonely prince. (Source: MangaPlaza) — Description from AniList community (anilist.co).