"Aus ist es mit der Diät."
Translation:The diet is over.
I would disagree. "So much with something" is not when you are done with something so much as when shrug something off. For example: When it rains and your umbrella breaks, "so much for the umbrella" or "so much for trying to stay dry". I would say "I'm done with the diet", or "I'm over this diet". Hope this makes sense?