remorse, regret, be sorry
憾 centers on a lingering painful feeling about something past: remorse, regret, or being sorry for what happened or what one did.
憾 combines the heart radical 忄 with 感, which likely contributes both sound and the idea of feeling deeply. The character expresses a painful emotion felt in the heart.
The heart radical 忄 beside 感 (emotion, feeling) suggests a heavy feeling in the heart: regret and remorse weigh on you.
For カン, imagine a can of bitter memories that you regret opening: can -> カン, and the contents fill your heart with remorse.
regrettable; unsatisfactory; deplorable; lamentable
fully; to the full; (most) satisfactorily; amply; sufficiently; completely; perfectly
to regret
I regret to say; I'm sorry to say; unfortunately
deeply regrettable
regret; disappointment
extremely regrettable; deeply unfortunate; most disappointing; utterly deplorable