What I remember of the story is that Luna found out that ponies died in her fight with Celestia as they wrecked the castle, she goes back in time to try and change things, but fails to convince Celestia to listen, can't find her past self in the cave she had retreated to, and when she tries to keep a certain pony away, he shows up to save Luna from being crushed by the rubble of the battle. This guard pony was an ancestor to Prince Blueblood, and when Luna says something like "You were supposed to stay away" he responds before he dies "Good thing I didn't" meaning in order to save her.

I tried looking for it every way I could think of, but no luck.

I haven't read a story like that yet, but it sounds like a really good one!

I can't say i have read this one but i would be really interested if you find it.

I have read one that seems like it might be based on a similar idea but it is celestia who goes back in time to change the outcome of the battle with Nightmare Moon.
https://www.fimfiction.net/story/539785/only-a-few-moments