If your horse won't do what you have asked, you keep asking until he does. That's the fundamental. You ask , you tell, you demand, though. So, your "asking" becomes firmer.
However, if the horse is not understanding your request, you have to be a bit more patient. But the idea is still the same.
So, horse is going forward, you get it to slow down, using a falling tone of voice, a lowering of your body, an exhaling of the energy from your body, and if that doesn't work, a few tugs on the rope line. When horse slows, you stop all those signals and do nothing.
Same for stop. You ask for the halt with your voice ( I am not huge on verbal cues, so dont' know why you want to have lunging be based so much on verbal cues, but anyway . . .), you put a bit of a tug on the line, then if that doesnt work, you put some real tugs on the line, and if you have to, you step in front of the horse (not smack in front, of course). If horse stops , but just whirls around the other direction, you keep up your "asking" . You continue to apply the signals for stop, namely the tugs on the line. If he whirls again, you keep it up. You do not give the release until he makes the correct choice. If he stops facing the wrong direction, no matter. He stopped, you reward. Just keep in mind exactly what you are looking for and reward only that.
If you are asking for too much and the hrose cannot understand, then break it down into several smaller steps, and reward for each of those when the horse correctly chooses and does these. Make it successful, but don't reward his wrong choices.