I Think Alot Of Things She's Been Threw Are Bad, Finding Out Serena Slept With Nate, Serena Was More Apperciated Than Blair, Chucks Pain When His Fater Died,Waiting For Chuck To Confess His Love For Her Aswell Was Quite Upsetting. :( x

But i think the fact that Serena was always more appreciated than she was, was also one of her nightmares.. But the whole love-hate relationship with Chuck. waiting for him to confess his love is def. her worst nightmare.

The whole love- hate thing with Chuck is a nightmare also. But Serena was the sth else; her mother, Nate, her friends, even Yale's dean prefered her over Blair. Her conversation in 2x6 with the dean revealed how bad B was feeling about all this. Imagine Little Miss Perfect, the "it-girl" to be your best friend! Eww! Poor B! i totally understand her.
(I don't have anything against S and I understand that she doesn't mean any of these. it just happens, :()

or the serena thing, shes always so insecure when it comes to that.

and Serena sleeping with Nate + family problems.