1. I'm sad.

2. I'm unhappy.

3. I'm in bad/dark mood.

△如果说又生气了，还可以用到mood的相关短语：

be in one of your moods，表示“又心情不好；又生气”。

Tim's in one of his moods so I'm keeping out of his way.

蒂姆又闹情绪了，所以我就离他远点儿。

4. I'm in low spirits.

△low spirits就是“无精打采；沮丧，忧郁”。如果说lift sb's spirits，就是“提高（某人）的兴致”。

Nothing - not even the prospect of dinner - could lift his spirits.

什么都提不起他的兴致，甚至连吃饭也不例外。

5. I feel so down.

△这里的down是形容词，意思是“不高兴的；提不起精神的”。

I've been (feeling) a little bit down this week.

这周我有点情绪低落。

【拓展】

down还可以做名词哦，比如have a down on sb就表示“不喜欢…，对…有偏见”。

Why do you have a down on him? I think he seems really nice.

你为什么不喜欢他？我觉得他非常不错。