Wasureru Kimi to no 300-nichi

Overview

Each day, Rihito confesses to Arata, and their relationship begins anew. At the end of each day, Arata forgets. One year ago, a strange comet passed over Earth, leaving countless individuals with inexplicable memory loss... One such individual was Arata. Not only did he lose his memories of falling in love with his childhood friend, but also the ability to make new ones. Unwilling to give up hope, Rihito logs each day they spend together by the sea and refuses to leave Arata's side—even if it means abandoning his own future. When each new day dawns with hope and ends with heartbreak, how many days can a forgotten love endure...? (Source: TOKYOPOP) Note: Includes two extra chapters.

Source: AniList