kǔ sī
to think hard
bitter thoughts
to pour out one's sufferings
kǔ sī míng xiǎng
to consider from all angles (idiom); to think hard
to rack one's brains
yì kǔ sī tián
to view one's past as miserable and one's present as happy (idiom)
láo xīn kǔ sī
to rack one's brains
to think hard