I've said mindfulness is about paying attention, which is a whole different bucket of fish from the idea of 'focus'. To most of us they are more or less the same.
Focus is when we're cocooned in that single minded pin-point concentration lasting a limited time before you start to think about the urgency of picking up your dry cleaning or watering that plant. But while you're focused, all those usual niggling nags of the to do list fade into the ether and self-consciousness ceases. I live for that sensation when I'm not just an old sack humped over a computer banging out random words; it's when I feel the computer and I are one, working together for a common goal. When I hit that state where I lose all sense of self, I'm in some pretentious people call 'the zone.' That means we're functioning at our optimum when the chore is just challenging enough to keep our interest but not so easy to be bored. People say it feels effortless, liberating, superhuman; You stay focused because each time you achieve a little more success in your chore, your brain releases a hit of dopamine. It's that little leap of joy in your heart when you splash into the pool after doing a double back, jack-knife, triple somersault off the high dive with both toes perfectly pointed. If it didn't feel good you wouldn't keep doing it. You'd think, "What the hell am I doing up here going off the deep end?"
So it's not the perfect swan dive but the hits of dopamine you're after. If after your efforts you happen to win a gold medal, trophy, bonus, hit a bulls-eye you'll get such a main line smack of dopamine you'll probably be addicted forever and spend your life hunting down the next hit at any cost.
As humans we're natural born addicts when it comes to some of our neurotransmitters, such as dopamine, adrenaline, noradrenalin. The more we get the more we want. The problem with focus is we can't tell where our tipping point is; when we're really accomplishing a goal or learning a skill and when it's just about getting another fix of dopamine. I'm guilty of this syndrome where I can't stop writing, it's midnight I'm dripping in sweat and I can't pull away from the computer. If I could, I might notice that I'm not making sense anymore, I'm writing in circles and a child of eight would tear up the work, it's so bad. But as long as that dopamine keeps flowing I keep going. I'm not even aware that my petrol has run out, that I'm running on empty.
We need to learn the lesson that if we keep pushing past our peak we'll eventually flunk the test, lose the job, miss the deadline. That's our punishment for beating ourselves like slaves until we collapse in the mud, dead.