A nose chain should never be placed under the chin. The reason being if placed there the horse could respond by throwing it's head up or even rearing if used even slightly. Also the nose chain should be brought over and under the nose of the halter one time. The first thing I saw was that your horse's hind legs were not squared up evenly. That is something you can work on at home in practice sessions. In your practice sessions to get the horse to square up behind is to back the horse a step or two until the hind legs are even. To square up the front legs use the lead to bring which ever leg needs to come forward. Also if you didn't clip the hairs at the coronet band of the hooves that would give a more well turned out look. For muddy conditions the hooves and legs could be gone over with a damp cloth or stiff brush to remove some or most of the mud just before entering the ring/arena. For your horse's mane and tail you can "bang" the tail just enough to get it all one length. The mane would look better either banded or beaded. Banding would probably be easier with your horse's thick mane. There are banding rubber bands and banding tools available at Tack Stores or in Catalogs.
The reply posted with video of how to present your horse to a judge is very good and informative to watch. The easiest way to remember where you position yourself for a viewing judge is to *never* get between your horse and the judge. Always present your horse between you and the judge.
Most of all, don't let your disappointment in your placing at that show stop you from keeping at doing it. Here are some