When Winston begins to believe O'Brien is lying, he sets out to practice

Winston doesn't change much, that's, in a nutshell, human tragedy.

Explanation:

Athough very deep inside Winston is hearing a little voice in his head saying that O'Brien might be lying, he is caught by the overwhelming human desire and need to trust somebody, especially in a totalitarian world like that of 1984. Moreover, the promise of O'Brien to introduce Winston to the Brotherhood makes it even more difficult to not getting caught in O'Brien's web.