Winston's fatalism is an aspect of his realism: he knows that eventually the state will catch up with him and punish him (probably kill him) for his various thought crimes. From the moment he starts his journal, he knows it is all over. His "crimes," such as having an affair with Julia, simply progress from there.
It shows the power of the state that Winston is so determinedly fatalistic. He thinks about "when" he is caught, not "if" he is caught. The Party has completely convinced him that it is all-powerful and that nobody escapes its all-seeing, all-knowing grasp. While he would like to believe he and Julia could continue to go on as they have, he has been indoctrinated to believe this is impossible.
What is interesting is not that he is fatalistic but that he actually underestimates the reach of the state and its all-encompassing presence. He knows his capture and death are inevitable, but he doesn't realize, for example, that Mr. Charrington works for the Thought Police or that the...
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