From Manga Abyss:

Things lost, Born anew, as love. The most painful love story in the world. “If it’s only for half a year, I’ll date you.” Kasuya was a difficult partner, Mio suggested that kind of relationship. It was supposed to be a real game plan, but Tomadou Kasuya became serious right away. Before long though, Mio found out that she has “juvenile Alzheimer” disease…