
Author's name is Michika you can check on the following links:
https://www.mangaupdates.com/series/6othjyn/konse-wa-hitori-de-ikiyou-to-omoimasu-sono-hazu-ga
https://kakuyomu.jp/works/16817330667229020968

The flow of story is not good, maybe it's the translation's fault. Topics keeps skipping from one to another. Some words are redundant or without cohesiveness, some parts are illogical esp to be very knowledgeable with just based on previously read books without having that particular book at hand (does she have a hyper photographic memory?).

Cheryl, the princess of a fallen country, was cursed by Piany, the princess of the enemy country that destroyed her country, with a curse that would burn her body if exposed to sunlight, and she spent her days being tortured. However, Cheryl was protected by Liam, a handsome general who came to occupy the royal castle, and as an efficient way to break the curse, she decided to kiss Liam morning and night to inject his magical power into her…!? This kiss was necessary to break the curse. But that was all it was supposed to be–

Read up to chapter 20 before reading my comments below
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Why didn't the prince gave the full information MC needed to become the real saintess of their country or to win the high saintess rank if his marriage, political power and kingdom's future is on the line?!?!
She keeps asking questions and you always brush it off with we'll discuss it later without doing so!! Information is the greatest advantage specially if your enemy is a very powerful & influential person with whom even the king is now in the palm of his hands!!!
This story's direction really pisses me off!!ヽ(`Д´)ノ

There's a plot twist (not including what I have written below).
They're in mutual love.
They're each other's first. ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄

I gave it a try and got some clues along the way that ML's action have reason to be like that, the grand reveal made me very surprised but it made me say "I thought so and that's the reason why.."
Anyway he's loyal to the point of almost endangering his life just to fulfill his promise to FL. Give it a try, there's no prolonged drama or cheating so far in 6 chapters~
Repost of:
https://www.mangago.zone/read-manga/the_young_lady_whose_fianc_cheated_on_her_is_doted_on_by_the_tough_leader_of_a_foreign_country_she_was_cursed_to_turn_into_a_cat_but_she_gets_petted_every_day/