I have a different theory that I'd like to put out there:
Winston, as the only (self-identified) human left, died in room 101. Rats were scarier to him than death itself, so he chose to "die" then. What did he have left to lose? From this point on he could not resemble what he considered to be human, His choice was rats plus more torture and inevitable death, or death. He played the game, and was tested when he was reunited with Julia. I believe that after his release he frequented the chestnut tree to await his physical death (having witnessed executions there previously).
So, not wanting to risk returning to Room 101 (because of the power of the party to see everything) I think he knew that there was some truth in what O'Brien had said to him, of existence and the mind. He had never been in control of his thoughts and his feelings, not until he left room 101. I believe this is where he "appeared" to sacrifice his freedom of thought, even to the reader. Every thought prior to room 101 was reactive, stubborn, naive etc. and not thought out to its end, in some ways his mind was never really in his total control. Until now. Now, he knew to mind his thoughts, or face rats.
Before his death he was able to push threatening thoughts from his head. He dismisses his "daydream" of his mother, because while it feels happy and comforting in theory, it does not exist now in reality. What good is it to ponder the validity of it, the warm experience of the thought is enough, it's truth is of no importance to his present reality. O'Brien had taught him that. "They did not matter so long as one knew them for what they were. Some things had happened, others had not happened." If you can understand this, then the end can be seen very differently.
I don't think there was victory for Oceania, O'Brien had earlier told him that there would always be war. I think by this stage he was hallucinating while dying, he was creating his last experience of existence/thought, like it was his final act of freedom, his rebellion.
I also think that he chose to love big brother, not in an act of submission, but for himself to die in peace with love. To die feeling human. See, he did not change, or feel forgiveness for himself (hence his return to the Ministry of Love in his dying moments), he didn't even love big brother, not until after the shot was fired. - I thought this was what prompted the line in the book, "He had started as though a pin had run into him."
So in my interpretation Winston won, not for us as readers, not for the humans of Oceania, he won for himself, he'd conquered his own mind, and so was able to fool the powerful Party. Freedom granted by his oppressors, on his own terms. He protected the one thing he most treasured, until his death. The white knight fell with forgiveness, love, pride, victorious, a noble death during the battle, even if he was the only one who thought so. This is Winston's story. This is the power of one mind. It also just happens to be a warning to us, the readers. Check your mental health :p because the world will always be problematic.