here's what I see. First your reins are too tight, and your hands are too low. Lift your shoulders up and back, and that will bring your hands up. Your reins actually need to be shorter with more bend in your elbows and your hands higher and following your horse's mouth. All in all, your hands should be 3-6" apart, and about 3" above the withers.
Next your back is very hollow and you are overarched, riding on your pelvic bone. That alone will cause a horse to hollow as it's putting your weight in the wrong spots and forcing your horse to drop his weight on the fore. Once your horse is out of balance, there will continue to be resistance on the reins and he will have a hollow back and false frame.
In addition to correcting your hands/shoulders, you need to also relax your seat, open your pelvic angle (slightly different than your hip angle as I understand for hunters you want a more closed hip) and relax into your seat bones. You also need to ride from leg to seat to hand, not the reverse which you are doing here. You are riding hand to leg, and have lost your seat entirely.
To work on relaxing your lower back, try no stirrup and no rein work on a longe. Then focus on where your hands should be sans reins so that you don't have to worry about trying to set the horse's head. One of the biggest faults I see these days is riders worried about headset, when they SHOULD be worried about footfall and driving the horse from behind. Your horse's lagging back, hollow topline, and pinned ears all indicate that he's not liking his job in that picture, and I would also venture a guess that in addtion to you dropping your pelvis forward, you're also causing issues with the saddle pinching. Be sure to have saddle fit checked by a professional, they can also evaluate how your position is affection saddle fit.
Good luck - and relax your back otherwise your horse will never engage his!