…And never get a chance
A chance denied by yourself
To fit inside the others story
To make it story of your own

@CaliperClick
It looks like, in this story, Sunset isn’t driven by ambition but by feelings of inadequacy. She looks in the mirror and doesn’t see a destiny that Celestia denies her but a goal she can never achieve. Note that she starts writing her farewell letter before Cadance pops up; the magic duel is a desperate last attempt to justify staying, and given the power she puts into that blast (as Butters Stotch noted), she knew she was going to fail.

Sunset’s decision making seems s bit off to me. i would expect her to become more emotional, angry maybe?
Mad at Celestia that she (subjectively) can not notice how MUCH more effort she (Sunset) is putting in, just to get low attention while Cadence gets wings and is accepted as niece seemingly out of nowhere.
Maybe she should confront Celestia personally, for drama’s sake?
Later she would ran off to prove a point or show that she IS the worthy one.
But here she’s just… i dunno… “you’re new princess? duel! i lost? i’m not good enough, too sad, unfair, wunaway in tears. well, off to find a way to become destined queen, bye”
maybe i didn’t get it?

Better origin than the comics.

Sunset you literally destroyed a mountain. If Cadance didn’t deflect that she would have died.

This backstory looks somewhat better than the story from IDW

If this ever goes to print, I need this.

Magnificent.

cant say i didn’t see this coming.

unu