to be ashamed
怍 centers on the feeling of shame or embarrassment, specifically the internal emotional state of being ashamed.
怍 combines the heart radical 忄 with 乍, which likely contributes a phonetic element. The heart radical points to an emotional state, specifically shame.
The heart radical 忄 next to 乍 suggests a sudden, sharp feeling in the heart: the pang of shame when you realize you've done something wrong.
For サク, imagine feeling so ashamed you want to hide in a sack: sack -> サク, and your heart sinks with embarrassment.