break_on_match controls behaviour when you are matching a field against an array of patterns. If it is set to false then grok will only match the first entry in the array that matches, and ignore subsequent patterns. You only have a single pattern, so it has no effect.
I would suggest consuming the entire file as a single event. Then run grok against that, or you might need to use ruby to scan the [message] field and pick out the first match from the array of matches that scan returns.