World Beyblade Organization by Fighting Spirits Inc.

To keep his childhood friend Mayuri "Mayushii" Shiina from dying and save himself from the emotional pain of constant repetition, Rintaro Okabe must move from the α (Alpha) World Line (a parallel universe in time) to the β (Beta) World Line. Having saved Mayuri over and over, only to watch her die in different ways than before, and a day later than the last, Okabe nearly loses it when he realizes that he must return to that β World Line, where one of his closest friends he meets since the start of the anime, Makise Kurisu, is stabbed and killed.

The pain becomes too much as he tries his best to save Mayuri without having Kurisu to die. But he is pinned against a wall, and Kurisu confronts him about his indecision. Prior to this, however, he had to erase the "D-Mails" that altered the α World Line in time he had sent, and in turn destroy each of the senders' reasons for doing it in the first place. In one instance, it was to save her father from dying, and another was to be Okabe's girlfriend as a girl than a really effeminate boy. One of the most notable lines in Okabe's talk with Kurisu coincides with a theory I came up with before I saw this episode, but hearing him say it brought me to tears.