run out of steam
US /rʌn aʊt əv stiːm/
UK /rʌn aʊt əv stiːm/

1.
勢いを失う, 疲れ果てる, 気力がなくなる
to lose energy or enthusiasm and stop doing something, or to become tired and less effective
:
•
The project started strong, but it seems to be running out of steam now.
プロジェクトは好調に始まったが、今は勢いを失っているようだ。
•
After working for ten hours, I started to run out of steam.
10時間働いた後、私は疲れ果てた。