Is "life is hard without jam" in use?

I think the underlying sentiment is probably the same as the English saying All work and no play makes Jack a dull boy, which usually means you need to indulge yourself sometimes.

I'd volunteer "A spoonful of sugar makes the medicine go down" from the film Mary Poppins. The medicine would a metaphor for life, in this case, and serves as a tone-setting statement for the film's theme that life's little pleasures should be enjoyed whenever possible to offset all of the troubles that come with living.

EDIT: Another option would be "Stop and smell the roses" which was the title of a Ringo Starr album in 1981, but could also be attributed a misquote of the golfer Walter Hagen when he stated, "You're only here for a short visit. Don't hurry. Don't worry. And be sure to smell the flowers along the way."

I've translated it for years as: Life is smelly without jelly.

I think the meaning of the saying is clear - the figurative role of confiture is similar to that in "always jam tomorrow" in English.

That said, it's not a standard phrase in English, and it's clear that the appeal of the phrase in French owes something to the rhyme between dure and confiture.

Perhaps the closest phrase in English is "A little bit of what you fancy does you good", but it's not a homely or comforting phrase - if anything it has an air of decadence.

Over all, I would suggest that you use a translation of the long form, which preserves the rhyming element in "but with butter it's better".

How about "What's life without luxuries"? One of my favourite sayings :)