Well your thinking about what "you" might be doing wrong and that's great! I would suggest losing the prong, you did something wrong or you still would not be having this issue. Been there did that, the only dog I had that would not walk worth a crap on a leash was the only dog I used a prong collar on incorrectly!
Since you seem to have both time and patience, just start over and give this a try:
Stop your dog from pulling on the leash video - YouTube
It will do no harm and might be all you need? If it fails then hire a trainer.