Akuyaku Reijou ga Sei Heroine o Kudoki Otosu Hanashi.

From Yen Press: When the crown prince announces he's breaking off his engagement to the infamous Akuya, his spurned fiancée wastes no time turning her seductive words of passion to her actual target—Sei, a commoner who worked her way up through the ranks with her wit and charm alone…and the very girl the prince dumped Akuya for! Sei's about to get a taste of what a real lover is like in this steamy tale of two girls who traded in their Prince Charming for each other! Note: The series has been publishing by the author on Pixiv, Twitter as a doujin. Made into a collected book officially by Enterbrain in 2022.