@Mio: They were basically just brainwashed and never
actually acted badly of their own free will. When Twilight got Discorded, it was
entirely her choice, which meant she'd feel a lot more responsibility. The other thing is that while the others did interact with each other's Discorded versions, they only did so after being being brainwashed themselves. Twilight had to deal with all of her friends acting like jerks to her at the same time, with no explanation as to why.
While I'm sure the others took a while to recover, Twilight definitely got the worst of it, as Discord was specifically attempting to break her.
And besides
that, none of the others have quite the same issues with living up to their own standards that Twilight does. RD has self esteem issues, and AJ really values her reputation around town, but in both of their cases they seek approval from friends and peers (RD's Wonderbolt aspirations aside) and the majority of the time they aren't being examined over it.
On the other hand, Twilight is basically the #1 student
in the country, and has probably been constantly graded and analyzed for most of her life. The closest thing she has to a vocation is
learning, so when she fails at that it hits her every bit as hard as it would hit RD to crash and burn in front of the Wonderbolts. Add in that she's intensely reverential of Celestia and it's not hard to see why she'd have an inflated idea of how important the assignment is.
edited 22nd Aug '13 12:46:20 PM by JapaneseTeeth