to lose strength; to become weak; to disappear (of energy, drive, etc.)
to wither; to droop; to wilt
to feel demotivated; to lose interest; to become disappointed
批判を受けてやる気が萎えた。
My motivation withered after receiving criticism.
連日の猛暑でスタッフ全員の体力が萎えている。
The energy of all the staff is flagging because of the intense heat day after day.
困難の前で気力が萎えるときがある。
There are times when one's spirit falters in the face of difficulty.