My biggest critique is you have no base or weight in your lower leg/heel. Your foot is just resting in the stirrup and if I had to guess, may be sliding around there. The stirrup should be on the ball of your foot and your heel should be lower than your toe. The BEST BEST remedy for this I feel is spending alot of time in 2-point. Get up in 2-point (start at halt) and relax, and think of your weight going down into your lower leg and heel. In the beginning you can grab mane if you lose balance, but try to find the "hot spot" where your legs are UNDER you, and no matter what horsie does, if he were to go out from under you, you would land on your FEET upright.
Once you master this at the halt, do it at the walk...eventually letting go of mane, then trot, then canter. I spend my warm-up in two point. Once I feel a good "base", I keep my legs in SAME position and sit. I always think "Wow - do I ever feel secure". Your legs will get sore. In the beginning my back got sore because I realized I had ZERO core strength, and no base of support in my lower leg. All I can say is 2-point is your friend right now. You will get stronger and stronger.
You can also get into the "winged victory" position to get your legs under you. This position basically resembles when the lead character in "The Titanic" movie is on the front of the ship with her arms out :) Stand in your stirrups (ball of foot on stirrup). Lean upper body forward, straighten legs and push legs/feet BACK. You will be hovering over the cantle. Put your arms out (do this at halt) and find your balance point. Stay there for a while, eventually doing it at W/T (tie your reins or loop them through grab strap or have a friend lead you). Stay in winged victory and then SIT, leaving legs in position. Again you should feel nice and secure. I believe this is in the Sally Swift book (which I highly recommend).
I also think there is something weird with the sheepskin cover - something doesn't look right to me and I'd like to see you in the saddle without it to see how the saddle fits.
Your horse is adorable - I have a thing for greys :)