Typically when a child makes a mistake on a problem--even though they're told to just keep going--most of them can't mentally "just move on." Most will continue to think back to the mistake they made, wishing they could fix it. They'll shift nervously as their eyes dart back and forth between their current problem and the one they know they messed up on a few problems back.
And all the while, the timer is ticking away. And with every moment the child thinks back, a moment is stolen from the problem they're on currently. They're only allotted a certain amount of time to get through all the problems, and they can waste an extraordinary amount of time looking back.
It is distracting to dwell on the past.
But time is ticking. I've got things to accomplish today, and so do you. And we're allotted only a certain amount of time in which to accomplish them. And what an extraordinary amount of time can be wasted by looking back. We can allow what happened or what we did in the past to consume this day, to steal from the current task at hand. Or we can leave those covered-by-grace things in the past--and fully focus on where God has positioned us today.
"One thing I do:
Forgetting what is behind
and straining toward what is ahead,
I press on
toward the goal to win the prize
for which God has called me Heavenward
in Christ Jesus."