A half seat on course doesn't really bother me, especially for a pony rider (been there, done that, don't blame you). However, you do need to be a bit more active in your guidance. Work on a consistent forward pace. When you had it, you had nice distances, but when you didn't you chipped. I'm not a huge fan of the hunter thing of riding with your hands low. It causes the the rider to tip forward and consequently the horse. You need to rotate your balance back a bit and raise your hands, it should also help bring your horse on to the bit. As already stated, use those corners. I was always told to take advantage of the corners, not get them over with. Put your weight into your outside leg, lift that inside hand and bed the horse around the inside leg. Suddenly your pony will be able to take the longer strides rather than having to do the choppy little ones through the turns. Most of this is flat work that needs fixing (dressage really). I think you're a good rider for this level, but you need finesse to be competitive in further levels.
Usually a horse being quick at a canter when the rider is sitting is usually all mental on the rider's part. Try exhaling as you ask for the canter, gently pushing the pony into it, and keep breathing. Allow your leg to just swing and relax through the hip and knee. Usually it's tension in your seat that causes you to inadvertently push the horse faster.