It looks like to me you need to scoot back in your saddle and roll your thigh and knee on to the saddle, bend your knees, roll your shoulders back and pull your chest out of your belt.  From the look of the video it looks like your stirrups are too short but it could be looking that way because you are sitting to high on your saddle; also you to work on the placement of the iron on your foot.  Now us saddlebred riders we have to have all of this and post while having perfect and still legs, so there is no sympathy for you ha ha.  I went many days walking like a penguin so I could have perfect legs on horseback.  Below are pictures of what is desired and how to get this desired position.  Gaited horse saddle seat equitation is the same as 3 gaited saddle seat equitation so just think if you can look like these girls you might revolutionize your breed.

You want to bend your knee to get the straight line ear, should, hip, heel.  This will take work if your leg is not trained to do this but in time if you continue to practice your leg will be trained and it will be one less thing to think about.  To help with your legs you are going to want to 2 point as much as possible, if doing it correctly you will feel it in your thighs and stretching in your calf.

“Grip with the knees and the reply is to not to grip your knees, let me explain: knee grip or contact is certainly a primary necessity, but to command a rider to grip with the knees results in the riders pulling the knees directly into the saddle with the thigh muscles.  This is a very tiring effort.  In order to attain the proper knee contact which will permit the muscles to remain relaxed and workable, the position begins with the angle of the sole in the stirrup.  This stirrup iron hangs at a slight upward slant that requires the foot to assume the same tilt with the little toe somewhat higher than the ball of the foot.

Now in order to attain the necessary knee grip, we use the stirrup as a lever, and by swinging the heel slightly down and out, we find that the foot actually pushes the knee into position.  This is assured when we realize that the entire leg worked as one unit from the hip joint and that the knee and upper leg roll into the flap of the saddle. So many riders try to correct out turned toes by twisting the ankles.  Not only does this weaken the ankle buy it actually pulls the knee away from the saddle. 

The leg works as a single unit originating at the hip joint.  Often this idea of a single leg unit will be misunderstood, and the rider will “lock” the knee.  The beauty of the knee grip through the foot position is that it permits the greatest economy of effort with a maximum of control and balance.  Bobby Morrow, the great track star, explained his success as his ability to keep his muscles relaxed so that they are free to do their work.  This is certainly true in riding as well as it is in any sport.

Just how far the rider should swing his heel to the side must be determined individually.  Certainly the foot should swing out only enough to roll the knee and thigh into position.  Swinging wider than that is an affectation that results in awkwardness and ugliness.  A good personal check for the rider is to glance down the side of his leg and swing the heel down and out until he can see the entire side of the leg down to the sole of the boot.  To check the foot for the forward position it is generally practical to say that the stirrup leather hangs at a right angle to the ground.

There is a well known phrase in riding, “the seat must be independent of the hands.”  Certainly this is so but such phrase needs a lot of explanation for the young rider.  Literally it means that we must have such good posture and balance in the saddle that no matter what awkward motions the horse may go through we will be able to keep our balance without hanging on to the reins to maintain it. It also means that our hands must be so thoughtful flexible and adaptable the any head movement by the horse taking care of by the hands and will not pull the body out of position. 

It is clear that balance and control seat and hands are very dependent upon each other.  Balance starts with the foot in the stirrup and like the old spiritual, “the foot bone connected to the ankle bone the ankle bone connected to the shin bone, etc.,” your entire body balance depends upon the feet in the stirrups.  Happily, if the foot is right in the stirrup the knee will be right.  If the knee is correctly placed into the saddle, the thigh will be snug.  If the thigh is snug, the seat will be in the right place in the saddle.  So, you see, your position is truly built from the foot up.  How very important it is that we realize this!” (Crabtree 50 – 54)

Now looking at pictures of you do you know what you should be doing and how to fix the problems?  What other things do you notice about the other girls that might improve your appearance?