Winston starts as a very frustrated, discontented, bitter and scared man. He hides his true thoughts, rankles at Party activities and requirements, and seeks for independence in writing in a journal. Dreams of "the Golden Country" haunt him, a place where the earth is beautiful and filled with light and freedom. He longs for this, but doubts it will ever happen, or ever was in the past. He is seeking for a mentor, for someone to share his feelings with, but feeling isolated and alone.
He begins to ponder change as he wonders if O'Brien is a rebel at heart also. O'Brien gives him hope, but he doesn't know what to do about it. So, he continues in his state of angst for quite some time. Then, Julia changes everything. Julia helps Winston to feel validated in his feelings. He becomes confident again, enlivened, and purposeful. He is happy. He knows he is not alone in his resentment of the Party, and succeeds in rebelling in his own way. He is no longer afraid, and risks all. She gives him fulfillment and courage.
Julia's relationship with Winston gives him the courage and bravery to respond to O'Brien's query. He boldly goes to O'Brien's house, something he would have never dreamt of doing before. Now that he knows people like him exist (through Julia), he is more confident and assured.
Unfortunately, O'Brien is his downfall. He is captured and through torture, stripped of all of his pride, dignity, self-respect, independence, and privacy. They even invade his mind and heart, taking everything he holds dear there. He fights it for a long time, but eventually caves. At the end, he is a broken man who has no semblance of his former, stronger self. He is unrecognizable, even to himself. But, he accepts it.
I hope that those thoughts helped; good luck!