Take a deep breath and relax! I used to have the same shoulder and upper body tightness issues. My trainer's advice...don't get out of your seat and let yourself get left behind. It sounds odd and it's not pretty, but over such low fences you aren't hurting your horse (just make sure you let the reins slip through your hands when being left behind). If you do CT/XC, this will save you!
Your horse has nice form, but he will have a much nicer arc if you teach him to jump into your hands, not against your hands...btw, quite a nice looking horse you have!