Akuyaku Reijou ga Sei Heroine wo Kudoki Otosu Hanashi.

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!
(Source: Yen Press)