For starters, ignore anyone who may have told you to look up or past the jump. When approaching the jump, look at it until it goes out of view between your horse's eyes, then look ahead. That will help with some of your horse's distance judgement problems.
I don't know how you're riding on approach to the fences, but for the last 5 to 8 strides before the fence you should be sitting in the saddle in a 3 point or light seat. You want to sit tall, close your legs around your horse in a bear hug, sink your heels down as you come into 2 point at the base of the fence, and press your knuckles against the side of your horse's neck as you provide a release over the fence. If you have been approaching the fence already out of the saddle to date, it might feel weird to be sitting, but you will quickly find it gives you a much better communication with your horse. The good ones will wait for that big bear hug to tell them it's time to jump.
As for judging distances, place two poles on the ground about 48' apart. Canter over them a few times and count how many strides your horse takes between the poles. Once you figure out how many strides he gets within a certain distance, place the poles to that distance and first practice keeping a nice, even rhythm over them. Once that's cake, role the second pole our 1/2 a stride and practice switching between a forward distance and a collected one. It's not the easiest size, but once you get it, it will come easier.
Hope that helps. Have fun!
You just have to see your distance...you don't have to like it.