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.
I did drop the stirrups just one hole and it seems to help. Of course posting is a little harder when I go english but my seat does seem to be deeper. I am having to make a special effort to move my legs back. I think I must have gotten a little lazy. Working on the shoulders back too. I was wondering about trying out that "shoulders back" contraption I've seen some english riders use. I always felt like it was a bit like cheatin' but now I'm curious....
I mentioned to my trainer that I dropped my stirrups one hole and she said I actually can drop them another one, maybe even two but to work on my seat now and then drop them. I do have my legs back now so I want to get some pix posted for you all to compare soon. Maybe this weekend?