When you are feeling down or in a bad mood, you can express it in English in several ways. Here are some common phrases you can use to convey the feeling of "心情不好":
Feeling blue: This is a common idiom in English that means feeling sad or depressed. You can say "I'm feeling a bit blue today" to express that your mood is not good.
Down in the dumps: This is another idiom that means feeling unhappy or depressed. You can say "I'm feeling really down in the dumps" to convey that you are not in a good mood.
Out of sorts: This phrase is used to describe feeling slightly unwell or not quite oneself. You can say "I'm feeling a bit out of sorts today" to indicate that your mood is not great.
Having a rough day: This is a more direct way to say that you are not having a good day and your mood is not positive. You can say "I'm having a rough day" to express your feelings.
Feeling low: This phrase is used to describe feeling unhappy or lacking in energy. You can say "I'm feeling a bit low right now" to convey that your mood is not good.
It's important to remember that it's okay to not feel okay sometimes. If you are feeling down, it's important to take care of yourself and reach out to friends, family, or a professional for support.