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to think hard; to brood over; to worry too much (about); to torment oneself (with the thought of)
彼は失恋を思い詰めた。
He brooded over his heartbreak.
心配で彼女は思い詰めているようだ。
She seems to be worrying herself sick.
思い詰めるあまり、冷静な判断ができなくなった。
He brooded so much that he could no longer think clearly.