I don't think you need to drop your stirrups too much. When you get them too long, it's too hard to keep your leg on the horse and in position. The days of the straight leg and reaching for a stirrup are long gone, thankfully!
It looks like you are pushing back against the cantle a bit. Stand straight up in the saddle, then sit straight down without letting your body push back to the cantle. Opening up and relaxing your thighs a bit will help as well. It's very easy to get tightened up and pushed back onto the cantle.
Good job on the show! It looks like you had fun.