How does Winston's character change throughout the novel?

Winston starts the novel groping to find his humanity through writing in a journal, but at this point, he is largely dehumanized. This is made most clear by his thoughts about Julia. Before he even knows her name, he sees her in her red chastity belt, notes she is both beautiful and (he thinks) unavailable, and wants to rape and harm her. This shows how the alienation, hate, and violence encouraged by the state has seeped into his soul.
However, once he gets involved in an affair with Julia and gets to know her and love her, he begins to become a real human being again. He wants to protect Julia, not harm her. She matters to him very deeply, and he experiences great caring towards her.
As Winston gains some sense of humanity, he begins to remember more about his mother, another person—probably the last other person—to show him genuine love. He also is able to turn from viewing the proles with hatred and contempt to seeing the large older woman who hangs the wash outside the window of Mr. Charrington's shop as "beautiful."
Of course, O'Brien's goal will be to try to eradicate all this newfound humanity.