Shitsurengo, Kenaku datta Osananajimi ga Satougashi Mitai ni Amai: Bitter nochi Sugar

Yuu Sawatari, a high school boy, is sick after suffering a painful heartbreak. His nextdoor childhood friend, Cocoa Shirayuki, shows up to take care of him. From that day on, the distance between them grows closer... This is a tense, sweet, and slightly heartrending love story of a boy who has lost his love and his childhood friend whose love has long been unrequited.

(Source: Kodansha, translated)

Note: Chapter count includes 4 extra chapters.