"Can you think of a single degradation that has not happened to you?"
Winston had stopped weeping, though the tears were still oozing out of his eyes. He looked up at O'Brien.
"I have not betrayed Julia," he said […].
He had not stopped loving her; his feeling toward her had remained the same. (3.3.77-81, O’Brien to Winston)
Despite prolonged torture, Winston’s final act of rebellion is to hold on to his private loyalty to Julia; he refuses to betray her.
Quote 26
Suddenly he started up with a shock of horror. The sweat broke out on his backbone. He had heard himself cry aloud:
"Julia ! Julia! Julia, my love! Julia!" (3.4.20-21)
In a fit of rebellion and manifestation of private loyalty, Winston refuses to give up his ties to Julia.
Quote 27
"Do it to Julia! Do it to Julia! Not me! Julia! I don't care what you do to her. Tear her face off, strip her to the bones. Not me! Julia! Not me!" (3.5.24)
Faced with his biggest fear, Winston finally betrays his private loyalty to Julia.