How exciting for you! I hope you're looking forward to it.
I have to admit to being rather lazy and I never plait my horse. I just trim her mane so it's quite short and very straight. It saves worrying about plaits coming out or looking untidy and it relieves any unnecessary pressure, allowing me to concentrate on more important issues. It's perfectly acceptable to ride unplaited so perhaps you could too?
It took me a long time to stop putting pressure on myself to do well and to stop being tense during tests, but nowadays I go in with a smile on my face and just try to relax and imagine that I'm riding at home in my own arena.
Try to focus on accuracy when approaching / leaving or commencing movements at the letters and if your horse is on the lazy side, try to anticipate it breaking pace before you ask. Try not to 'over-ride' the walk too because that will just push the horse onto it's forehand and when doing free walk on a loose rein, remember to loosen your hips and allow them to move with the movement of the horse.
Most of all ENJOY IT! Don't get yourself stressed out. It's not worth it for the three or four minutes you're going to be in front of the judge and winning isn't the be-all and end-all. It's the taking part that matters.
Edited to say try doing lots and lots and lots of transitions to wake your horse up and get it going forwards more. Work on a 20m circle and alternate between a few strides of walk and a few strides of trot - on both reins. Then do the same working trot - canter - trot, plus lots of walk - halt, serpentines and trot - halt - trot if the horse (and you) are up to it. Work on the basis that when you ask the horse goes, without spending time thinking about it. If it doesn't respond to your leg aid, a firm flick with the whip just behind your leg should do the trick.