If you want help debugging a grok expression you increase your chances by providing a complete and low-friction example that people can use.
That said, narrowing down a huge expression is usually pretty simple. Start with something very simple and build it up from the start of the string, piece by piece. When you start getting the _grokparsefailure tag you've found the problematic part of the expression. In your case, start with something that matches an yyyy-mm-dd date and build from there.