During a conversation in French, I was wondering how I'd express the same idea in Russian:
Elle peut pas se morfondre et broyer du noir éternellement. {or: se morfondre dans le noir}
The "broyer du noir" originally referred to the way a painter intentionally ruins his/her own piece of work by smothering it with black paint – as you do when you're feeling downhearted. And now, the expression figuratively refers to those (temporarily) moping around with a negative outlook. As for the alternative "Se morfondre (dans le noir)", it corresponds to "mope around (in the dark)".
Не можем же мы все время ходить с постными лицами.
This phrasing comes to mind as its Russian equivalent, but I can't seem to find a dictionary entry for it, so I'm wondering if it might not be used that commonly to express this idea.