Winston changes dramatically throughout the novel. At the beginning, he is trying to survive in his society, but all the while doubtful, questioning, and hateful towards the powers that ran it. He knew things weren't right, but didn't quite know how. He knew that he couldn't be the only one that hated the party, but felt hopeless that he could connect with anyone else. He was confused and struggling.
As time passes, and he meets Julia, his confidence in his own mind and rebellion increase. He realizes that he is not alone in his feelings, and that life might be worth living, if only for a while. He enjoys life almost, and feels validated in his feelings against the party. His thoughts become more lucid and powerful; he is able to record more logical and thought-out analogies in his journal. He grows bolder in his attempts to evade the party, securing the room for himself and Julia.
After he is captured, he goes through a dramatic change. He mind and body are broken, and he comes out of the experience almost unrecognizable. For a long time, while he is tortured and questioned, he held on to his logic, his hatred, and his own thought processes, but in the end, they got that too. They taught him well that truth is only what the party says, even if it doesn't make sense. They taught him that the party must have his heart also, not just his thoughts. They taught him that he is a coward that would betray loved ones to save his own skin. He comes out a total party worshipper; he loves big brother, he loves the party. He drowns himself in gin and lives a meaningless life filled only with a boring job and a fixation on war, and he doesn't even think of living any other way. His independence, his mind, his hatred, and his agency is completely destroyed.
I hope that those thoughts helped; good luck!