I'm trying to find the name of a book I read about 15 years ago. The cover had a picture of a girl standing in front of a bunch of rockets.

The story was about a girl who finds out that a computer that was surgically implanted in her head is a prototype that allows for her brain to be broken into multiple personalities. 

The company or government (I don't remember which) is trying to capture her again but she escapes and gets help from a hacker who helps her unlock her true potential.