In chapter four of Book three Orwell writes,
"He remembered remembering contrary things, but those were false memories, products of self-deception."
This proves that he knew these thoughts he had previously had that went against the Party's teachings were untrue. (Although we audience members know they are true because we have just seen Winston transform before us.) He acknowledges that accepting the Party's principles was much easier.
In fact, as the storyline continues, he begins to embrace more fallacies as truth. He becomes taken with the idea that all happenings take place in the mind, and to be fully participative in the Party, he had to learn to control these flashbacks. He therefore developed an ability to block thoughts that could dangerously influence him. This new reflex he developed is called Crimestop.