This may seem crazy, but I honestly don't know exactly what a half halt is! I have a vague idea, but not enough of one to put it into practice. Could you expand on the process of a half halt?
"A half-halt is used to get a horse's attention, and to ask him to balance. You use a half-halt when you are about to ask the horse to do something: to go from a trot to a canter, to make a turn, speed up or slow down without changing gaits, or most anything.
The half-halt itself is not a request for a change of pace or direction. When you half-halt, you should not slow down, speed up, or make a transition; you keep going at the same gait and same pace you were going before. You just want the horse to be listening for you to ask him to speed up, slow down, transition, etc. Similarly, you don't want the horse to start to bend or weave around or wiggle in place. You want him to keep moving steadily forward just as he was doing, with his weight shifted a bit to his haunches to prepare for your signal.
How do you give a signal that asks the horse to just keep going, but to listen up? What you do is this: you ask him to slow, and to go, at the same time. Keeping contact on the reins, you squeeze your seat to ask him to slow down, and you squeeze your calves to ask him to move forward. In a "half-halt," you use all the signals you would use if you were asking the horse to whoa, but you stop asking before he slows down--"halfway" to the halt.
The amount of rein, seat, and leg you use will vary from horse to horse. Some horses will need more leg than seat, for example, and some horses may need you to squeeze the rein a little instead of just keeping the same contact. It shouldn't be hard to "feel" how a horse is responding, and adjust your signals."