You really need to pick your eyes up and look up. Look up ahead to where your going, and you need to be thinking about your next movement, preparing your horse for it. You don't need to see your horse to be able to know it is moving underneath you as your asking. You should be feeling your horse, instead. When you look up and concentrate on the movements ahead, you will be able to feel more 'inline' with the horse. You will feel when your mount is stretching through the back to the bit underneath you, or if she/he is tense in the back.
By bringing your eyes up, it will naturally bring your back up as well. You have seemed to loose the straight effective back in these pictures, and I'm guessing its because you've made a habit out of looking down. This happens, and if looking down is made a habit your back will roach, as it is in the pictures.
Bring your eyes up, and roll your shoulders back. Think about sitting deep in the saddle with your seat.
Also, when practicing this, bring your hands up, and thumbs on top.
The looking down and back roaching has made you fall on your hands and use them as your base of support. In turn, it has made your elbows pop away from your sides.
So, to sum it up, bring your eyes up, and your shoulders back. Sit deep in the saddle, and straighten your back. Don't let your shoulders get tense when you do this, make sure your back and shoulders are still relaxed, but in the position they need to be. Bring your hands up, and thumbs on top. Bring your elbows back to your sides, and ride with your elbows bent. This will help you when half-halting, and will give you the feeling that you have your whole forearm to half halt, instead of feeling like all the strength has to come from your hands alone.
Another BIG thing, is ride from your core muscles. Right now, I bet your back hurts when you get off the horse. You are using just your hands and arms, and that is going to make for a very sore back. If you sit up and use your seat and your ab muscles (your core) muscles, it will feel much better, and trust me, you will get much better responses from your horse.
Your horse looks to be trying to give, it has a nice gait, but it appears cracked in its back. It does not look completely through its back and stretching TO the bit like we want. This is more than likely due to the way you are sitting.
Once you correct your seat, I am almost 100% confident this horse will round underneath you nicely.
I have had the same problem in the past, so I completely know what you are going through. Just really concentrate, and you'll fix it in no time!
Good luck, and very nice mount!