Sugar After Bitter

Shitsurengo, Kenaku datta Osananajimi ga Satougashi Mitai ni Amai
失恋後、険悪だった幼なじみが砂糖菓子みたいに甘い

Source

After a Heartbreak, My Bitter Childhood Friend is now Sweet Like Sugar
Shitsurengo, Kenaku datta Osananajimi ga Satougashi Mitai ni Amai: Bitter nochi Sugar· 失恋後、険悪だった幼なじみが砂糖菓子みたいに甘い ~ビターのちシュガー~
novel

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.*