Seishun wa Sankakukei no Loop

Because I've been watching her more closely than anyone else, I could tell. That she's fallen in love. And that the one she's in love with... is my best friend—our mutual childhood friend. Determined to let go of his own feelings and support her love instead, the protagonist, Tsubaki, makes up his mind. Thanks to his efforts, the two childhood friends—always by each other's side—finally end up together. After seeing it all through, Tsubaki spends a sleepless night, choking on the urge to scream... And then, when he next opens his eyes, he finds himself back one week in the past, to the moment before he gave up on his feelings.

(Source: ComicWalker, edited)