At the end of Part 2, Chapter 7, Winston and Julia agree with one another on what the ultimate betrayal would be. They say that confessing what they have done would not be betrayal. The only thing that would be a true betrayal would be if the Party got them to stop loving one another. This would be the ultimate betrayal because it would mean that the Party has actually controlled their thoughts, not just their actions.
Winston's rebellion is meant to allow himself to be able to think and feel and have a true inner life that is independent from the Party. This is what he is trying to create for himself by writing in his diary and by carrying on the affair with Julia. This is his way of rebelling. Therefore, it makes sense that giving up on this would be the ultimate way to betray what they are doing. If he stops loving her, he has given up everything that he is working for. He will have given up his efforts to be a real human being with real human feelings.