Seijo Fuzai ni Yoru Kari Hajime Konna no ni, Bukiyou na Outaishi ni Dekiaisareteimasu

Lady Rosaria has memories of her past life, memories of the story she read that she is now living in. A saintess will appear to take away her fiancé the prince, and she will be exiled to a far away land... but the story isn't going to go that way if she can help it. Rosaria has been adamant about losing the prince's favor but the saintess simply does not appear... and now she and the prince are going to marry?! Now Rosaria's on a mission, get as prepared as possible for whenever this Saintess is gonna show up. Once she's here this marriage is done for! Although... it does seem like the prince feels a bit different from the story...? (Source: MangaPlaza)