I’m not in the mood
没有兴致，没有心情
She is not in the mood for talking to him about this topic.
她心情不好，不想跟他讨论这个话题。
I'm upset 心烦意乱
I'm still upset but I know it is for my own good.
是啊，我还在郁闷呢，不过我知道这是为了我好。
Heart sinks
小心脏往下一沉，可想而知有多伤心了。
My heart sank when I saw the amount of work waiting for me.
看到有那么多工作等着我做，我很不开心。
I'm down 心情低落
He is a very good listener and he knows how to cheer me up when I'm down.
他是个很棒的倾听者，每当我心情低落时，他知道如何让我振作起来。
In low spirits
心情很low，情绪低落，英文里用这个短语表示。反之，兴高采烈则是in high spirits。
when I am in low spirit, I always listen to this song, which always bring me new hope.
每当心情低落的时候，这首歌总是为我带来新的希望。
The art of being happy lies in the power of extracting happiness from common things.
快乐的艺术是由平凡提取快乐的能力。