How do I figure out how to correct my riding seat?

it looks to me like your stirrups are a little too long for flat riding. maybe try lengthening them a hole or two and sinking into the deep of the saddle. ride with your eyes closed. this can be very scary, but it will tell you how good your balance is :P it will tell you what side to work on. riding with always improve your balance and natural seat as well. hope this helps at all