Konyaku Haki sareta Kaedama Reijou, Hatsukoi no Toshiue Ouji ni Dekiai sareru @COMIC

Louise, a noble lady who loves reading and researching magic tools, leads a double life pretending to be her twin sister. Suddenly one day, her fiance breaks off their engagement and chooses to be with her sister instead. All seems lost to her when she also has to give up her dream of becoming a magic tool researcher, but a reunion with her first love, the first prince Lucius, saves her from her misery. As they spend time together reading books in the hidden archives and looking around at different magic tool shops, Louise slowly regains her confidence. On top of that, her heart flutters every time unconsciously sweet words leave the prince's mouth. The mood is perfect and it seems like it's seconds before they can finally be together, but the past she wants to keep hidden gets in the way. "How...do you feel about me?" He asks, but...I can't do it. I can't tell him. Meanwhile, her potential as a researcher begins to blossom and leads to...A true love fantasy full of misunderstandings between a magic tool-obsessed noble lady and a stoic older prince awaits! (Source: MangaPlaza)