I agree that he's too young to do much lunging. I believe that the slight problem my own horse has with one of her hips can be traced back to the extensive groundwork she went through as a youngster.
Anyway, it seems as if you're doing fine, but sending her out too far too soon. Maybe a more gradual progression is needed. You want to actually increase the amount of your space. Same as what you've been doing, just more so.
My trainer sends a horse out first by pointing, then with the other hand, making a big sweep with his arm, overhand, towards the horse's jaw. Then, he concentrates on driving from behind. Keep your body behind the girth-area.
And do go slowly! You'll be glad later.