[Matrixbeast]

Okie dokie lokie!

Nightmare Rarity spoilers below:
Basically, the Nightmare forces preyed on Rarity's personal self doubts (Specifically choosing to target Rarity) and weakness (Dash and AJ are incredibly athletic and strong, Fluttershy, despite being timid, will do anything for a friend, regardless how scared or at whatever risk to herself, Pinkie's eternal optimism keeps her level headed in a bad situation, and Twi is one of the strongest magic users in Equestria, leaving Rarity as the "Odd man out" so to speak), and overtook and convinced her that the other 5 would replace her with any other pony as both their friend and the element of Generosity the first chance they could.

This is what the Nightmare forces did to Luna a thousand years prior, taking Luna's own depression for how the other ponies reacted to her night and amplified it, convincing her that Celestia and the others didn't care for Luna.
This is where Sweetie Belle played a part. She didn't save Rarity, but Luna. When Nightmare Rarity came down to conquer Equestria, Luna felt it was her fault the Nightmare forces are even a threat and tried to confront them. It was through Sweetie Belle and Celestia that Luna learned that everypony loved her and weren't angry over her actions as Nightmare Moon.

Rarity was saved through the mane 6. The Nightmare forces convinced Rarity that the others saw her as worthless, and even when Rarity was separated from Nightmare Rarity (Thanks to the efforts of Spike, who went through hell and back to even get through to her), the self doubt was still clouding her mind. It took the remainder of the mane 6 to remind her that all her friends loved her, and that they'd never replace her through memories of good times they shared. This reinvigorated Rarity's spirit and removed the negative emotions and lies the Nightmare forces were feeding her to sustain itself. It was a rather endearing and touching scene. Brought a smile to my face.

So, in short, the Nightmare forces were hoisted by their own petard (I didn't get into that, but the Nightmare army itself was defeated through betrayal), Luna was saved by Sweetie Belle and Celestia, and Rarity was saved by her friends. I didn't detail everything Spike went through, but he was more of a main character in this arc than Twilight, which was also pretty cool.