Dreamquest by Brent Hartinger

Eleven-year-old Julie Fray is living a nightmare. Her parents are fighting so much she can’t even escape when she goes to sleep. Every night she has horrible dreams–and all she wants is for them to stop.
Then one night she wakes up inside her dream, in the studio where her dreams are produced! There is even a star who looks just like Julie who plays her in her dreams. With some help from a production assistant named Roman, Julie learns that if she can somehow find the people responsible for producing her dreams, she might be able to make them less scary. But it won’t be easy: the tiny tear in the “fabric”of her consciousness that let her fall into the dream studio in the first place is closing fast…and she could be trapped inside her own head forever!
Even worse, it turns out that when Julie fell into her dreams, Vivian, the actress who plays Julie in her dreams, escaped into the waking world–and the fake Julie will stop at nothing to take over Julie’s life.