Hi, I live in Ireland, and teach riding (it's all english riding here) I am not familiar with the standard there, but here, if i was giving you a lesson (and i mean this in as nice a way as possible) I would tell you that you are relying on your reins/or balancing on them a bit too much, your hands should just look a bit more relaxed. there should be a straight line at all times from your elbow, through your wrist and hands, along the reins right to the bit. Your hands could be a little lower, and shouldn't move up and down so much with your movement.
There should also be a straight line from your ear, through your shoulder, hips to your heel, so i cant tell really well from the short video, but i think you need to sit a bit more forward in the saddle. as you go up and donw, think more like forward and back with your belly button, instead of up and donw. when you land in the saddle you should sit in the middle of the saddle instead of the back of it, and you only need to rise a tiny amount for working trot (normal) that way when you want to get your horse working better and using himself more, you can use your seat-to rise a bit higher-and so to get a longer step from him. Again, i cannot tell much from the short video, but those are basic english positions as we use here and also in England.
Your horse is really pretty, good luck in your competition!!! I'm sure you'll do great.