Well yes truth is objective, but then everyone has a personal dealing with truth too. Willie, Magenta, you, and me can have a personal truth that is subjective to us and also objectively true too. For instance using math, objectively 2 + 2 = 4. Subjectively it is true to you that 2 + 2 will always equal 4. To me it is subjectively true that 2 + 2 = 99 - 95. Objectively the answer is still 4. Truth can be approached in many ways subjectively and still objectively yield the same result.
So for Winston his truth is subjective, he hates Big Brother, he hates the Party, he sees a lot of problems with the society he lives in even though others do not see the same as he does. Winston is not totally wrong either, in his subjective truth is objective truth that the Party is not good. Winston cannot express his subjective truth because that is in essence thoughtcrimes against the Party. Anyone whom expresses their subjective truth about the Party will be punished. The prole woman is a good example, subjectively true to her the scene at the movie of a dismembered child's arm was emotional and should not be shown to kids. The Party punishes her by banishing her from the movie theater and disregarding her thoughts and her individual personage as being typical prole behavior.
Heh I don't like math either though, math is mostly totally theoretical made purely to convince the world of lies, but that's for another topic.