Book: The Host by Stephenie Meyer
Song: Voice In My Head by Hilary Duff
It's not the whole song that fits the book, rather than just bits and pieces:
"The little voice in my head won't let me forget
The little voice in my head is never misled
All of this noise is what keeps me from making a mess
The little voice in my head just won't let me get with you"
Doesn't that sound like Mel when Wanda was around Jared? Or even Ian?