Sunset Shimmer would never break up with Marble Pie. Sure, the relationship could be going better—a lot better—but ending it would break Marble's heart. Only a really terrible person would do that, and Sunset is done being the bad guy.
Unfortunately, it looks like someone else has decided to be the bad guy for her.
This is an (ineligible) entry into my own contest.
Special thanks to Sam Rose for the amazing cover art, and to R5h for editing.