Ok, I can list a few causes of halitosis for you and see where you think the problem may be, we can then take it accordingly.
so halitosis can be caused by poor dental hygiene, dental carries... for this, it is usually recommended that you have scaling and polishing with your dentist every 6 months. sometimes there are residues left on the tongue, making a habit of regularly brushing the back of the tongue can help. oral hygiene is the commonest cause for halitosis.
stomach issues like acid reflux or H pylori bacteria, which can be tested further with a stool antigen test by your GP. If it does come out positive for h pylori then you may need a course of antibiotics. for acid reflux, the underlying cause should be seen and a course of PPI medications can help.
dehydration.. if you don't take enough fluids or if you take excessive tea or caffeine. Increasing fluid intake, sucking on lozenges and sugar-free gums which can increase salivation will help. also making use of mild mouthwash(alcohol-free ones) regularly will help the problem and not damage the oral mucosa.
stress, smoking any skin disorders and poor immune system can all contribute.