In English, the term "low mood" or "feeling down" can be used to describe the feeling of being emotionally low or depressed. The term comes from the idea of feeling emotionally "low" or down, as opposed to feeling happy or uplifted.

Here are some example sentences using these terms:

I've been feeling really down lately, so I'm going to try to do some activities that make me happy.

My friend has been in a low mood since she broke up with her boyfriend.

I always try to be positive, but sometimes it's hard when I'm in a really down mood.

In Chinese, these phrases can be translated as follows:

我最近感觉心情很低落，所以我要试着做一些能让我开心的事情。

我的朋友跟男朋友分手后一直心情很低落。

我总是尽量保持积极，但有时候当我心情很低落的时候就很难。