On a positive note,
Love your horse. Got to love blacks. ;) And I'm a sucker for big bushie tails. Your turn-out together is neat and tidy and looking quite clean! You guys look like a good pair.
On the critique side, when you execute your halts put the back of your hand on the small of your back. When you exhale, think of pushing that part of your back into your hands. You will feel that the point of your hip rotates slightly up to allow that part of your back to get "filled in". If you have back issues, then doing it this way will allow you to find that spot in your back without causing you discomfort, because it is subtle and a different way of breathing. Once you find that sweet spot you will be able to be more secure.
Next time you ride have someone come out with you and try something super cool to "test" if you will if you are secured in your position. Sit on your horse, hold your reins and have the helper stand in front of your horse and take both reins on both side of your horse's mouth. Have them lean back a little, and see if they can pull you forwards out of your saddle. Try not to resist too much, just see what happens. If your helper can pull you forward, or you find yourself really trying to stay seated, you are not grounded. Put your hand at the small of your back, exhale, and fill that space. Put your hands back on the reins and see how secure you are then. Keep exploring where it is, how straight you can make your back to where when your helper pulls on the reins you can easily remain seated and the pull grounds you to your saddle, and your horse takes a step forward. Once the energy moves through you and moves your horse underneath you from the pull of your helper, you know you have found your secure sweet spot in your back that will make riding a lot more simple, and safe!
Once you find that, go back to your halts and practice finding that spot in your back and explore what it does.
You look like a nice, competent rider you just need some fine tuning. Put a little bend in your elbows and remember to breath. :)
Let me know what that does for you. Its hard for me to help you fully over the computer but I will be curious to see if that helps you.
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.