Hello dear,
Since you say you have had a dental check up and there is nothing wrong with your teeth I assume there is no decay. Did your dentist also check for pockets in your gums ?
Next Steps
Practise tongue cleaning if you don’t do that already. Hope you don’t have swollen adenoids or tonsils they can also be cause for bad breath. Also drink water too often and squash it around your mouth while drinking. Even small sips will do but keep your mouth hydrated every ten minutes or so depending on your condition. Avoid foods which have odour like garlic , onions and other foods with ammonical odour.
Health Tips
Keep your mouth hydrated and use mouthwash regularly twice a day