I have been scowering everywhere and I can’t find this really good fic I read a long time ago. Basically cadence turns to sombra after getting a letter that shows shining armor cheating on her with another pony. Can I get some help finding it please?

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I believe the story you're looking for is one of Stormy787's. It's explicit, so I can't link it, but the description of it certainly fits.

Dude thank you so much!! @mayhammoth!!!