Here's a good philosophy. Rather than shoot for the hole, imagine or picture a set of railroad tracks running infinitely straight out from the dock. Your objective is to get the tandem wheels of the trailer onto those tracks as soon as possible (or farthest away from the door as feasible) only then orientate the tractor onto those same imaginary tracks as you SLOWLY proceed backing toward the dock. Do not forget to watch your trailer end as well. In other words when you are watching the wheels or tandems of the trailer, about every 1/2 second, glance up from the tandems and see where the back of the trailer is going. Once you get partially in line, be careful of the swing of the front of the trailer at the fifth wheel. If you've gone too far past your railroad tracks and try to cut back there is a danger of over compensating at the fifth wheel and you could hit the tractor of the truck next to you.
You'll get better with time. Practise in a vacant truck stop at the back of the lot is a great suggestion.