Jipchak Sijongeuro Saranamgi

A reader is magically transported into a BL novel called The Night of Hwangseong. She now inhabits the body of Lobelia Flore, the daughter of a noble family suffering from a rare illness. Lobelia discovers that the only way to avoid a painful death is to remain in close physical contact with Edric Dencart, the young, beautiful, and sickly duke. To do this, Lobelia disguises herself as a man named “Lobel” and commits herself to becoming the Duke of Dencart’s most trusted manservant.
(Source: Tapas)
Note: Chapter count includes a prologue.