Konyaku Haki wo Neratte Kioku Soushitsu no Furi wo Shitara, Sokkenai Taido datta Konyakusha ga "Kioku wo Ushinau Mae no Kimi wa, Ore ni Betabore datta" to iu, Tondemonai Uso wo Tsuki Hajimeta

Viola wants nothing more than to break off her dull arranged engagement with Phillip. He's the son of a duke, and Viola is thoroughly convinced she's an unsuitable future wife for a man of his station. Worst of all, he seems utterly uninterested in her. So when Viola is in a carriage accident, she takes the opportunity to fake amnesia in hopes of getting out of her betrothal. Little does she know Phillip is also an opportunist, and he takes advantage of the situation by feeding the “amnesiac” Viola a tale she can scarcely believe—that they've been passionately in love this entire time!
Now trapped in her own lie, Viola is forced to play along. Can this curious couple outwit each other into happiness? Or will their bizarre web of deception get them both caught?
(Source: J-Novel Club)