恨 (Kanji for hold a grudge)

Onyomi

KON	I hold a grudge against the pine CONES that fell from the tree and knocked my pops on the head.

It's good for your heart to hold a grudge. 4 of 5 bitter doctors recommend it.

うら*む	to hold a long-term, soul-eating, ulcer-making grudge- against the bitch who stole your boyfriend, etc. ★★★☆☆

痛恨(つうこん)

deep , bitter regret ☆☆☆☆☆
痛 (hurts!!!) + 恨 (hold a grudge) = 痛恨 (deep , bitter regret)

	Meaning	Hint	Radical
恨	hold a grudge	HEART	心
根	root	TREE	木

Simple!
You hold a grudge in your HEART,

but the TREE has a root.