Winston, now released from prison, has become an alcoholic. He has been given a job editing the Eleventh Edition of the Newspeak dictionary, haggling over details with other bureaucrats. The rest of the time he spends drinking gin at the Chestnut Tree Café, worrying about the progress of the war, watching the telescreen, and playing chess alone. On the dusty table, he traces "2 + 2 = 5."
In learning that, when facing the ultimate torture, he would sacrifice even the thing he loves most, Winston loses not just his sense of self-respect but even his sense of self. He fills that hole with alcohol. As O'Brien put it, he has become one with the Party.
He recently ran into Julia, on a cold winter day in the Park. Her body had thickened and stiffened, and the thought of having sex with her filled him with horror. She admitted that she betrayed him under torture, and he admitted that he betrayed her as well. They parted, agreeing to meet again but with no intent to actually do so. Winston, deeply ashamed, returned to the café to drink.
Having lost their self-respect, they have lost the ability to love, or even lust, after each other. They have no private lives to share, nothing to excite them. They know they are owned body and soul by the party.
Winston hears a melody and the lyrics "Under the spreading chestnut tree / I sold you and you sold me," which he remembers hearing when he saw Jones, Aaronson, and Rutherford sitting in the same cafe so many years earlier. He begins to cry, and remembers a happy time when he played Snakes and Ladders with his mother. He quickly dismisses this as a false memory that never happened, and returns his attention to the telescreen, which is announcing a great military victory in Africa. Winston is overjoyed at the Party's triumph. He imagines himself back in the Ministry of Love, the hoped-for bullet entering his brain as he walks down a corridor. He looks up at the portrait of Big Brother on the wall, which fills him with a sense of happiness and safety. He knows that the struggle is over: at last, he loves Big Brother.
The lullaby Winston was remembering is about Room 101, about the betrayal–of those they love and of themselves– that all people will perform in the face of the ultimate torture. Winston practices doublethink to escape any thoughts of his mother or his history, denying himself by denying his past. The Party doesn't have to try to control Winston's reality. He controls it himself, and in doing so keeps himself as one with the Party. He has lost himself. He has given himself to Big Brother.