Note how Orwell doesn't describe to the reader exactly what kind of smile was hidden beneath the dark moustache. Not being precise in the description leaves it open to interpretation.

I imagined that it was a kind of smugness, pride, victorious. I believe Winston had felt the power of control, he understood that feeling now, he'd mastered his own mind just like BB had mastered the minds of the masses. So then he's incredulous, amazed that with his success, he found that he had much in common with BB, he's almost thankful and indebted to BB.

I can totally understand your position, because "self-willed exile from the loving breast!" Was actually what I found challenging in my interpretation. But then I reflected on my own life, and I guess I'll have to tell you a bit about myself here.

I have lived with an insane person, their scattered thoughts sometimes overwhelmed me. I was naive, young and very impressionable. I didn't know that he was insane for many years. Nor did he. I could've almost lost all sense of my reality and allowed myself to be absorbed by him. But I found strength in my mind. I found myself, more so than I could've without the experience. So I am grateful for that very dark period of my life. I love and hate this person. I would never want to go through it again, once is plenty. I would never wish it upon anyone else. But I wouldn't never change it. Because I am absolutely in love with my mind, I love it's capabilities, its strengths, I know it's weaknesses, I know it can be challenged to the brink of wanting self destruction and still come back stronger. Anyway I learned to deal with his insanity, to keep peace.

If Winston had control of his mind his entire life, such as the power he had in the moments leading to his death. If he'd known that his perception alone is all that mattered (just as BB would have everyone believe of him), then there would have been no need for all the misunderstanding, the stubbornness, the exile. Winston could have had his cake and eat it too, as such. They both could have been victors, masters of minds.

Note also that in my interpretation he was shot BEFORE he loved big brother, O'Brien had told him he won't be shot until AFTER he loves BB. Isn't this an indication of his victory?