the saddle IS a horrible fit for you. it is too big, by a fair amount. if you raised your stirrup, which you will find stabilizes your seat a ton, your stirrup iron might actually start interfereing against the bottom edge of the saddle flap. that saddle is a no go.
find another saddle.
What Tinyliny said...
Someone mentioned to shorten your stirrups, I think the length is fine, considering that you are rising well above the pomel at the posting trot. In dressage you don't need to rise as much at the post, in fact the closer you remain to the saddle the better. This allows far better contact when riding a dressage test and more advanced moves. That's why upper level riders sit all the time. The stirrup length might seem long because you are rather tense or contracted throughout your body. I don't know how to describe this, maybe someone else could throw in a better explanation. You are not fully stretching down and you are not completely balance. Relax throughout your body and improve your overall elasticity by working on a two point and stretching that heel down.
A good saddle makes a difference, but asides from that, what you reaaaally need is to improve your seat before you begin to work on anything else. You will highly benefit from working witouth stirrups while someone else lunges your horse. Focus on using your hip bones to absorb the impact of the trot and to allow your body to relax and reach further down without losing form in your upper body.
I do a lot of jumpers, so I ride far more often with a forward seat. But whenever I go back to dressage I ask someone to lunge the horse while I ride whit a crop behind my back. My elbows have to hold it in place and this forces me to stretch my upper body, straighten my back, open my shoulders and open my thoracic cavity more. Also while I am doing that I will work at the trot and change between sitting and posting trot. I will drop my irons for a few circles and post without them. Have the person on the ground make sure you look up and are not tipping forward at all. Wish I had pictures to illustrate this torturous procedure. It's a great workout and does wonders when I go back to a dressage saddle in a very small amount of time. I also become impaired for a few days from all the soreness lol
Hope this helps !
Edit Add: My indicator of long stirrups while riding dressage is when a rider struggles to get above the seat and is quickly sucked back into the saddle. Then yes, the stirrups are long. Good stirrup length allows the rider to post and have about a hand of space between you and the saddle when rising. Which OP can do by looking at the pictures. Otherwise if the rider is able to rise, but is having trouble reaching the stirrups, then that is mostly an indicator that the rider is tense and "short" or curled up in a "ball" instead of stretching down. If you shorten the stirrups, you won't be really addressing the problem and in my opinion it can become counter-beneficial in some cases. Try working on stretching out more, and if you still feel unbalanced or having a terrible time reaching down you can readjust.