Does your horse stretch his neck down forward to the ground or suck behind the bit and put his head to his chest? Does he keep the rein contact or do your reins go slack when he does it?
A horse that has formerly been trained to go behind the vertical is very hard to fix. First make sure the bit is not causing problems. Perhaps it is a loose ring that is pinching, or a mouth piece that is too fat. Anything the horse doesn't like about the bit might be the source of the problem. Once the bit is ruled out as a problem, proceed onto retraining. First ditch all tie downs. Forget about his head and neck for now. You want the horse to stretch forward to fix this. On light contact, pick up a trot and just ride forward, forward, forward. Once you have forward, incorporate large circles. The natural bend of the circle allows the horse to balance on the outside rein start to take proper contact. Any time the horse reverts back to sucking back, transition to walk and right back to trot.
While you're doing all this, be sure to keep you contact, light, consistent, and elastic. Put your hands out in front of the pommel with your elbows hanging at your side, and keep them there. At no point should you be pulling back. We are all guilty of it, but just start trying to be aware of what your hands are doing. You want your horse to like the contact he is feeling and want to stretch into it.
Sorry I wrote a book. Hope it makes sense and helps.
You just have to see your distance...you don't have to like it.