I doubt after so long you're still on it, but just in case and for other people who would land here:
1) use a save manager app on your console like Checkpoint to keep your save where you ruined your town (this is backup 1 and you shoudn't touch it anymore)
2) Save a second file of this save with your save manager(this is backup 2)
3) open ACNL and select "new game", then you play until you can save (you can try and retry again until the map is interesting enough)
4) backup this save into a new file in your save manager (this is backup 3)
5) use your save editor (probably
https://www.marcrobledo.com/acnl-editor/ ) and load backup 2.
6) click "import a map" and then load the garden_plus of backup 3.
7) move manually everything because this doesn't save the buildings and ressources's positions and you wouldn't want your house in the middle of the river, would you?
8) save it in backup 2's file
9) restore backup 2 into your game using your save manager in your console
10) if the save is said "corrupted", then you'll be happy to have backup 1, restore that one and overwrite backup 2, then try again.
Note: when you're totally sure that your edited save works right, you can overwrite save 1 with it. You should make two backups whenever you're going to do something potentially destructive for your save, in any game.