run out of steam

(verb) to lose energy or interest

Example Sentences:

A: Are you still enjoying your Art class?
B: No, I’m really running out of steam. The teacher is boring and I hate art.
A: Perhaps you should’ve chosen a different course.

After running for 10 km, Robert started to run out of steam and had to take a break.

Tiffany ran out of steam while studying for her final exam, and fell asleep at her desk.

From the Blogs:

Has Sonic finally run out of steam

Media:

Similar idioms:

1. run out of gas
2. blow off some steam
3. let off some steam
4. run across (2)
5. run across
6. run over
7. run of the mill