It’s never easy to get along with one’s parents. It’s even harder when they gave you way and denied you your birthright. This is the story of Sunset Shimmer and how she fell out with and returned to speaking terms with her mother Princess Celestia of Equestria.

This happens between Rainbow Rocks and Friendship games. Human Twilight (Or Science Twilight, whichever you prefer) would have add a whole layer of awkward.
Annual 2013 “The Fall of Sunset Shimmer” is also cannon to this but I cover all you need to know to follow the story in the prolog.