I suffered from the same problem since childhood. I tried everything. Every toothpaste and mouthwash sold, all kinds of doctors, even had my tonsils removed. Nothing helped.
Then I visited my ENT for a sleep problem and during a routine examination she found a nodule on my thyroid. She removed it two weeks ago. It had, apparently, been growing below the surface of my neck for YEARS. It was not visible from the outside. When the doctor removed it she said it was the size of a baseball, largest nodule she had ever seen.
My breath has been normal since she removed the nodule. IT was causing the smell.
Hope this works for you.