Where is self-betrayal in 1984?

Betraying others is an essential survival strategy in the totalitarian state of Oceania. The Party doesn't want you to trust anyone else. That way, instead of turning to your family and fellow citizens for support, you turn to the Party. The Party is always right, and only the Party can be trusted. As everyone is under suspicion, you can never be sure that someone won't turn you in to the authorities to save their own skin. In such a situation, there's no choice but to get your retaliation in first; to betray them before they betray you.
This is the toxic environment in which the people of Oceania are forced to live. As well as betraying others, it's often necessary to betray yourself. Whatever you personally feel about the regime, you need to suppress it as best you can and convince yourself that any problems you encounter are your fault and not the Party's.
In book 3, chapter 5, that's the situation in which Winston finds himself. In betraying Julia under torture, he is also betraying himself. In his very depths, Winston hates Big Brother and everything he represents. But his spirit has been utterly destroyed, and so he emerges from the horrors of Room 101 a broken man, ready to begin his reintegration into society.