Winston has a strong fatalistic streak, which we could also understand as realistic streak: he is smart enough to understand that the Party can tolerate no deviance. From the start of the novel, from buying a journal to opening the journal and writing the first word, Winston knows he has crossed a line that can't be uncrossed. He has transgressed, and the state will interpret that as the right to kill him. He knows the power of the state and its surveillance and that it will inevitably catch him.
Having started down this path, Winston's only hope is that somehow the rumored underground working to overthrow the Party is real. He is O'Brien's "last man," a person who can't turn away from his convictions and his humanity until these are forcibly stripped from him.