Complex sentence: Ramona reveals that she always hated Gail, even when they were supposedly friends, because she always competed with and outdid her, culminating with Gail taking the boy that Ramona wanted to the prom.
Split into simpler sentences: Ramona reveals that she always had contempt for Gail, despite the fact that they were friends, because she always competed with and outdid her. But she became enemies with Gail when she took the boy that Ramona wanted to the prom.