this is an alternate sequel to gutovi's too late
https://www.fimfiction.net/story/163898/too-late

rainbow dash succumbs to disharmony after she asked for forgiveness to applejack, and ran away from equestria and she ended up back to the privious land that the ponies used to live before they founded equestria and meets somepony watching her.

the co writter is AzuraKeres