In mindfulness meditation as it's typically taught, the point of focusing on the breath isn't just to focus on your breath. It's to stabilize your mind, to free it of its normal preoccupation so you can observe things that are happening in a clear, unhurried, less reactive way. And "things that are happening" emphatically includes things that are happening inside your mind. Feelings arise within you - sadness, anxiety, annoyance, relief, joy - and you try to experience them from a different vantage point than as usual, neither cling to the good feelings nor running away from the bad ones, but rather just experiencing them straightforwardly and observing them. This altered perspective can be the beginning of a fundamental and enduring change in your relationship with your feelings; you can, if all goes well, cease to be their slave.
After devoting some attention to the overcaffeinated feeling in my jaw [while on a meditation retreat], I suddenly had an angle on my interior life that I'd never had before. I remember thinking something like, "Yes this grinding sensation is still there - the sensation I typically define as unpleasant. but that sensation is down there in my jaw, and that's not where I am. I'm up here in my head." I was no longer identifying with the feeling; I was viewing it objectively, I guess you could say. In the space of a moment it had lost its grip on me. It was a very strange thing to have an unpleasant feeling cease to be unpleasant without really going away.
There is a paradox here. (Don't say I didn't warn you!) When I first expanded my attention to encompass the obnoxiously intrusive jaw-grinding sensation, this involved relaxing my resistance to the sensation. I was, in a sense, accepting, even embracing a feeling that I had been trying to keep it a distance. But the result of this closer proximity to the feeling was to acquire a kind of distance from it - a certain degree of detachment (or, as some meditation teachers prefer, for somewhat technical reasons, to put it, "nonattachment"). This is something that can happen again and again via meditation: accepting, even embracing, and unpleasant feeling can give you a critical distance from it that winds up diminishing the unpleasantness.