Starting Over by Sugaru Miaki

Translated from Japanese Completed Romance Time Travel

My second life began at Christmas when I was ten. I'd been given a chance to do it all over again, yet much as I racked my brain, there wasn't a single thing I wanted to redo. My wish was just to re-enact my first life exactly. But while I thought I was being accurate, things gradually slipped away. As if I were paying the tab for my all-too-happy first life, I was heading rapidly downward....Then at age 18, in spring, I met my "substitute". A substitute who, as I lived a second life entirely estranged, was faithfully living my first life in my place.

It's been too long since we took the time
No-one's to blame, I know time flies so quickly
But when I see you darling
It's like we both are falling in love again
It'll be just like starting over, starting over

- (Just Like) Starting Over
John Lennon

This is the story of how upon reaching my twentieth birthday, I was sent back to the age of ten, and lived to be twenty once more.

1: Starting Over