I'd say there were a few things going on here. Its really hard to reach a standard in any subject without regular practice. The run up to a flight test needs to be a consolidaton period and you need to fly regularly every day prior ideally for a week. I did my commercial in similar circumstances and I cancelled my own check ride and came back for another 10 day period despite the cost. It turned out to be a good decision. Another factor is the theory - your comment about lack of syllabus is a telling one. I suspect you are under prepared on that. The Thom books, whilst factual are quite hard to study from alone. Might be worth looking at alterniatives such as the FAA downloadable books. Some of the airlaw is diferent but the principles of flight and other stuff is really good in these books ( and they are a complete syllabus). Its also quite hard to learn and get good at things like flight planning and weather decisions from a book. You should be able to plan a flight from scratch in an hour but you'll never do that unless you practice. You should be watching the weather for the week before your skills test anyway ( personal minima) so that should be in your head before you get to the airfield. As for performance - you can easily get a leg up on that. You should know/ask the examiners weight beforehand and you can easily work out a no-wind runway length based on that weight at say 20 deg C and your likely field elevations before you start. That will enable you to easily qualify most destination runways without further calculation. Book mark the AIS notams page and look at them the night before for likely test route. Print off the destination airflield VFR plates with taxiway names and stuff and have them ready. Get used to looking at synoptic charts and the F215 - you can do that any time.
Hope that helps?