How do I control a horse from pulling down on the bit while I'm trying to halt?

you probably need a little more bit and try working on exercises that make your horse back off of the bit like doing a course at the add step or whenever your horse pulls on the bit stop him/her and back your horse up then go forward again and restart.
Personally, that sounds to me as though the mouth piece to your bit isn't right for your horse. My pony started doing this, would pull against it until I gave up and let her walk. We changed bit and she changed attitude.
Try half- halting before you ask the horse for a full halt. That burst of pressure and relse will refocus him and get him form pulling in the bit
Hope this helps
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