Wasurena Tsuma no Kimi to

Kazuki and Towako met in their high school days and were together for 55 years. Kazuki, who cared for Towako who had dementia, was so sad that he tried to commit suicide by drowning with his wife's body. When Kazuki woke up, he was not in the afterlife but in the world of his second year of high school. Though he was confused by the time leap, Kazuki approached his classmate Towako in the hopes of being united with her again...

(Source: Natalie, translated)