The show I am trying to identify shares some characteristics with The Girl Who Leapt Through Time, namely the elements of time travel and romance. The plot of the show that I am trying to identify involved a girl who was in love with a boy in her class. One day, she overheard the boy jokingly say to his friends that he preferred older women. The girl, not realizing this statement was a joke, somehow goes back in time in her current form, and subsequently, the rest of the world's people are a few years younger.
When the girl finds the boy's whereabouts, she realizes that he does not recognize her, in fact, the boy cannot sense her presence at all, if I recall correctly. She realizes that she has become a mere observer of the world of a few years past. Years pass, and it is revealed that the boy is still interested in pursuing a relationship with the girl, but is not aware of her whereabouts. I think, after high school, he goes abroad to study. The girl is unable to communicate to the boy that she is the person he is searching for, for the fear that the rift in space-time caused by her attempting to interact with the past world will destroy her, and thus, becomes heart-broken. She leads a sad, regretful life, never getting the chance to tell the boy her feelings for him.