Do you know why the log is being marked as grokparse failure? Maybe we could help you solve that issue.
If you are dead set on splitting up your log, how are you reading the log in? Through a file? Syslog? If you are reading a the log in using the file input that has a configurable option called delimiter which allows you to set a new line delimiter.
Perhaps you could attach a sample log, the grok you constructed and your logstash.conf file, removing any sensitive information.