Does winston really own the space in his own mind?

That is a good question. If we are to believe O'Brien only the party creates reality. Winston tries to hang on to independent thought but O'Brien holds all the cards. He tortures Winston and breaks his will. Then he fries Winston's brain like scrambled eggs until Winston succumbs to the Party's reality. In the end, Winston cannot control his consciousness: In the end, Winston must concede that 2+2=5.