Don't tell her jokes or funny things at this time, so she will be more sad, maybe she will think you are gloating; Even if she laughs for a while, she will still feel empty and sad afterwards.

In addition, don't say anything that makes her more sad, and don't scold her, which is equivalent to making her commit suicide. In a word, don't let her have emotional conflicts and extremes, let her be calm.

Then say what to do as a boyfriend.

Spending more time with her, talking less and taking more walks will make her feel relaxed. If she is interested in chatting with you, you must cater to her and talk more. If she is silent, you can tentatively find some topics. If she doesn't want to say more, then say less. Also, you can make out with her properly and give her a warm feeling. When the whole world is so dark, you are still with her. Generally, girls' psychology is so subtle, but don't be too affectionate. It is best to hold hands or give a big hug.

I believe it won't be long before her mood will get better.