This Is My Second and Final First Love

Kotoe, the heroine, nicknamed "Nodding-chan" for going with the flow and living a life full of regret, one day witnesses her boyfriend of three years cheating on her with her best friend from high school, Airi. In addition to the betrayal of her beloved best friend, she learns that all the misfortunes that have befallen her were Airi's doing. As she's overcome with despair, she's hit by a car and time-leaps 15 years into the past. It was spring of her second year of high school, when she was 17 and seriously in love with her first love, Minato.