Applejack and Luna have been dating for some time, and in that time Applejack has watched as Luna tore herself up inside over long held frustrations that never waned. Luna does her best to control those emotions, locking them away in the recesses of her mind, but that may be doing more harm than good.
Applejack wants Luna to air her frustrations before they air themselves, but after so long, Luna simply isn't able to. Every part of her that could, all assertiveness and confidence and anything else that made Nightmare Moon who she was, is locked away too deep to dig out. As she is, Luna has no choice but to bottle it all up.
So, it's time for Nightmare Moon to make a return.
This is set in the same universe as my short story anthology, but it'll stand entirely on its own. There's no need to read any of those other stories, but they are there if you're interested!