can't see the forest for the trees; lose sight of the big picture by focusing on details
Proverb used when someone is so preoccupied with small details that they fail to grasp the overall situation or larger context.
You're so focused on the numbers that you can't see the forest for the trees.
木を見て森を見ずというように、彼は全体の計画を忘れてしまった。
As the saying goes, he couldn't see the forest for the trees and forgot the overall plan.