This is a very , very common proglem with lunging. How do you get the horse to move away from you such that you can get access to his driveline.
What's a driveline? This is the position on the horse's body, about where his girth would go, that if you put pressure in front of that point, he will retreat away backward or sideways away from it. IF you put pressure behind this point, he will (or should) react by moving forward, away from the pressure.
So, you want him to go forward, you need to be able to get access to his driveline area and put pressure on BEHIND that point. If he is facingt you, you cannot get access to his side. If you try to walk around to access it, he will just turn and continue to face you while he swings his hind end around and away from you. Continue in this way and He'll be lunging you, around and around. Good workout for YOU!
So, you must make him move his head and shoulders away from you so that his side becomes accessable. To do this, yes shorten the line a little. Like to about 6 feet between you and the horse. Then you start to think about moving his head away from you. Stand with your body and especially feet pointed slightly in the direction you are going to have him walk. Now, start to put pressure on his closest nostril. You will begin by tapping the air with your fingers, literally pushing the air toward him. This will eventually be your sole cue. Of course, he will not move. Start to backthis up with swinging the whip. Or I use the end of the leadline and "propellor" it . Then , keeping my eye and focus on that inside nostril, I walk toward him and put pressure on his cheek and neck with the rope. I may get so close that the propellor with strike his neck. He'll jump and probably step sideways away from me, which will bring his drivelin into view.
In the beginning, I would want only this; a step sideways away from my pressure and so would give a release of pressure and a verbal praise.
After he gets better at this, when he steps over, you pull the leadline a bit forward and change the pressure from his neck area to his barrel area or even hip and say "walk on !" as he goes forward, you also will walk a small circle. You must be very careful not to get in front of his shoulder (driveline). He will stop and face you if you do, and it'll be YOUR fault, not his.
Practice moving his shoulder away from you so you can access his driveline. And then get him to walk on. Whenthis gets good, you can go to a longer and longer line, though I never use more than about 10 feet between me and the horse.