How Starlight Glimmer Actually Reformed

Twilight successfully reformed another villain but not the way we thought it was done.

Based off of this Comic from doubleWbrothers
The beginning of the Daughter Starlight Timeline

Starlight Glimmer caused some troubling problems with Twilight and her friends. What past caused this pony to go down such a path? How did Twilight fix such a horrible past to make this pony better?