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one's heart; the mind; inner feelings; inmost thoughts; true motives
「心中」 has another reading with different meanings.
心中を察してもらえると嬉しいです。
I'd be glad if you could get my feelings.
彼女の心中は複雑なものがあったようだ。
She seemed to have complex feelings inside.
心中穏やかではいられない状況だった。
It was a situation where I couldn't stay calm inside.