As a few other people said your hands move too much. Make sure your elbows bend slightly when you post. Also, it looks like you elbows may be sticking out a bit. Try to keep them closer to your body. The posting does look a bit forced but as someone else said it's probably because the horse trots very slowly. It seems almost collected.
I found a way to see how you look while riding is to watch your shadow. I haven't taken lessons in years so I watch how I move in my shadow to help. I was doing the same thing with my hands and it has really helped me correct this.
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