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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.

I'd say it's rather с кислыми минами, though yours is close enough too.

постное лицо (literally "the fasting face") is a smug, pious expression.

This is a face someone would put on (mostly hypocritically) to show the others that it's not time to have fun, and while the others might be having fun, they (the one with the face) certainly don't.

I just learned that there is a dated English expression "a Good Friday face" which seems to convey the same meaning.

кислая мина (literally, "the sour face") is a passive-agressive expression of discontent. This is a face someone would put on to show they don't like what's going on and they don't want to be a part of it.

Постное лицо is to show that you're better than the others, кислая мина is to show that you don't like what's going on.

Also как в воду опущенные.

In principle

Не можем же мы постоянно быть/ходить как в воду опущенные

can be said, however normally this idiom features in nominal sentences

And как пришибленные, this one is very vernacular.

As a variant - с траурными лицами

broyer du noir
In a high register it is предаваться мрачным мыслям, but there's a good Russian verb for it хандрить.

Короче русская хандра им овладела...(Пушкин)