I'll Be Your Mirror (Reflect What You Are) by cellardarlings
Summary:
When Ben disappears on Exegol, Rey refuses to let him go. Instead she follows their bond to try and bring him back from the beyond, and the Force is helpful in the way the Force often is: it sends her back to the day they met. She didn't intend to wake up strapped into the interrogation chair on Starkiller Base, but if this is how she gets Ben Solo back, then this is how she gets Ben Solo back.
Notes:
Inspired by @iamthesenate's tweet: "AU WHERE REY TRAVELS BACK IN TIME TO PREVENT BEN'S DEATH BUT ACCIDENTALLY TRAVELS TOO FAR BACK TO TFA SO YOU HAVE A BESOTTED REY AND A CONFUSED DARK LORD WANNABE BEN SOLO"
I originally threw the idea around with my beta as a smutty joke and then it developed an actual plot. Word of warning: don't ever feed plot bunnies, even in jest, unless you're prepared to feed, house, and care for them.
Thanks to my aforementioned beta, reylonging on Tumblr, for her help in shaping this. My writing wouldn't be what it is without her input and feedback.