"Lack of focus when practicing comes from one of two things: boredom or frustration."
If you are bored, raise your standards. That side of the equation is very straightforward (although not always easy!)
If you are frustrated, break the passage down to its individual parts (fingers, ear, rhythms, range, dynamics, etc) and figure out exactly what requires attention.
If a passage is in the upper register, it may appear that range is the reason you are missing a lot of notes. But if your fingers are close but not exactly correct, you will continue to miss those notes until you clean up the fingers.
So continuing to hang out in your extreme high register with sloppy fingers will not only not fix the problem, it will tire you and only reinforce the bad fingers leading to even more work later on.
There is a sweet spot that lies between boredom and frustration. The best players in the world are also the best practicers. They have found a way to hang out in between the boredom and frustration and get more done in less time than those who don't.
More done in the practice room in less time? Sign me up!