What becomes Winston's only concern?
I think that you are talking about what happens to Winston Smith's mind as he is being tortured. I think that you are talking, specifically, about something that is said in Chapter 19, or Part 3, Chapter 2.
In that chapter, we are told that Winston sole concern is to figure out what it is that they want him to confess to. He no longer cares about anything else, so he confesses to all sorts of things that he has never done, including ones that they surely know he hadn't done. All he wants is for the torture to stop.
His sole concern was to find out what they wanted him to confess, and then confess it quickly, before the bullying started anew.
Love overcomes Winston Smith's better judgement once he meets Julia in George Orwell's novel, 1984. Winston's previous marriage was an unhappy and ultimately unsuccessful one, and he had few thoughts of women until the day he received the note from the mysterious Julia. Her rampant disobedience of party rules only made Winston more interested, and after several meetings, Winston was hooked. He undertook the desperately dangerous act of renting a love nest for secret meetings which eventually led to his capture and torture. Still, he held out far longer than expected before giving Julia up; O'Brien tells him later that Julia had betrayed him much earlier.