This quote comes from Chapter 13, and refers directly to Valentine. At this point in the story, she and Peter have gained such a large following on the "nets" as Locke and Demosthenes, that even their own father is talking about them at home.
In many ways, this quote refers to Valentine's fear of becoming like Peter. Part of her initial partnership with him was an effort to control him, or at the very least, prevent him from controlling her. But here, Valentine is willing to admit she is actually enjoying writing as Demosthenes, enjoying the feeling of power, and enjoying the challenge. She second guesses these feelings because she inherently disagrees with the somewhat paranoid, anti-Russian author she is pretending to be. She is afraid that her thinking (as Valentine) will begin to change because of how fully she must engross herself in the personality of Demosthenes in order to continue on her (his) current path of success.
This fear, and this quote, indirectly relate to Ender in Battle School. The more he experiences success by figuring out and manipulating the system, the more he fears that he's merely becoming exactly what the system is creating of him. He considers the teachers the enemy. He is aware of their manipulation, he is aware that their agenda is much bigger than him (as an individual), and he feels like the "training" is turning him into a robotic monster.
But he cannot help but want to continue to succeed. His success means he's wearing an identity of a "soldier" or a "leader" who no longer looks like the child he was growing up. He fears that in each celebrated win, he loses a little more empathy and ability to be human.