When I watched Magical Mystery Cure for the first god-knows-how-many times, there was a detail I failed to notice. Twilight suddenly knowing what to do and glowing was always weird to me, until I compared it with what happens to the remane five shortly afterwards.
Finding the Part of Themselves That's Been Lost
Both Twilight and the Remane Five chose to try and help their friends, even if it's beyond their abilities to do so.
"My Friends!" / "My Destiny!"
Both Twilight and the Remane Five glow with a brilliant light. (BONUS: Twilight becomes the Princess of Friendship, meaning that, in a way, she also realizes her destiny here.)
Both Twilight and the Remane Five have things appear in their eyes. (BONUS! Both culminate in something to do with cutie marks: Twilight's cutie mark appears in her own eyes, and the Remane Five's cutie marks reappear on their flanks.)