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to feel down; to feel sad; to be depressed; to be in low spirits
to be in a slump (business, economy, etc.); to be in an unfavourable condition
to fall into (e.g. a hole)
試験に落ちてしばらくの間ひどく落ちこんでいた。
After failing the exam, I was deeply depressed for a while.
友人に裏切られて彼女はすっかり落ちこんでしまった。
She was completely dejected after being betrayed by a friend.
落ちこんでいる時は好きな音楽を聴くようにしている。
When I feel down, I try to listen to music I like.