After Winston Smith is imprisoned, during the interrogation, O’Brien urges him to understand self-betrayal as an element of the conversion that will succeed only because Winston himself embraces it. In part 3, chapter 2, O’Brien tells him,
When finally you surrender to us, it must be of your own free will…. We convert [the heretic]… we bring him over to our side, not in appearance, but genuinely, heart and soul.
When Winston admits he has neared the breaking point, he clings to the idea that he has not betrayed Julia. By this, he means, “He had not stopped loving her” (part 3, chapter 3). That point comes when he is shut into Room 101 with the rats.
The fundamental issue is not the impact on Julia when Winston betrays her. Instead, the real effect is on Winston, because he realizes that love is meaningless to him. He betrays himself by acknowledging that he is not a good and generous person but one who values life more than integrity. The self-loathing that comes with this betrayal is a necessary step in his rehabilitation as a Party member. Only after Winston hits rock bottom can he start to rebuild the ability to form an emotional bond with someone else. The new connection, however, will not be with a specific human being but with Big Brother as the embodiment of the Party. He will embrace the vision that O’Brien lays out:
There will be no loyalty, except loyalty toward the Party. There will be no love, except the love of Big Brother.
The last few pages of the book show Winston going through the final steps of that conversion, and the book ends with his epiphany: “He loved Big Brother.”