"I pretty much" can be used when you ALMOST did/said something, or also used if you actually DID do somethig, (can be a little exaggerated sometimes) depends on the conversation, I guess. Ex: "I pretty much ate the whole pizza by myself" "I pretty much cried in front of the class" "I pretty much died of hunger" And "all at once" is for when a lot of things happen at the same time or everyone does something at the same time... "All at once everyone started crying" "All at once the boxes fell off the shelf" Hope that helps a little bit :)

"Pretty much" is a colloquial American expression that means "more or less," as in "approximately." All at once," in this context, means "suddenly."

"Pretty much" means "mostly". "All at once" is the same as "suddenly".