Well, I am new here, but I have been lurking for a while.
For a little background, I have spent a lifetime training western horses (and a few Huntseat and Dressage horses years ago). Balanced stops and light compliant back-ups have always been very important to everything I have trained. Without them, you are just pulling around on a horse and he will never be 'really trained'. As long as there is resistance in a stop or back-up, you do not have a balanced horse that is 'using' himself properly. The elevated head, bulging lower side of his neck, hollow back and hind end dragging out behind him are all testimony to his lack of collection and balance while going forward.
I never ask for a stop or a back-up (other than a step or two ) until I have a horse droping is nose, coming back to my hand at all times and very 'soft' in the bridle while going forward. At least, going forward, I can push a horse up into the bridle until he gives me his face. I worry more about getting him to lift his back and his belly, stretch his topline and about keeping his shoulders up. This is the meaning of true collection - whether you are riding English or Western. Without it, stops and back-ups will always be horrible.
You only have a good stop when the horse is trying to stop and the rider is only 'asking' him to stop. If you have to pull on a horse, YOU are trying to do the stopping and the horse is trying to go forward.
Same can be said for the back-up. It is only good when the horse is picking his feet up diagonally and HE is lightly stepping back. If his feet are dragging, YOU are trying to do the work and YOU are trying to back him up. It is never going to be light, willing or pretty if you are doing that.
You need to drive this horse forward into the bridle until he drops his nose, relaxes his jaw and quits pushing on you. Then, you need to give him the reward of NOT PULLING on him. Make him carry himself -- you can't carry him anyway.