I was wondering if diet and exercise are the main key to eliminate persistent bad breath?

Nope: Bad breath may be caused by dental or medical issues or both. Other than not chewing on garlic, etc. Diet should have little effect. Exercise will also not eliminate bad breath, in and of itself. Speak to your dentist first and physician if necessary in order to determine the cause. The cause will determine the way to prevent it. See: http://smilesapartcosmeticdentist.Com/fresh-breath-control.Htm.

No: Use dental floss between your teeth every few hours and run it across the back of your tongue to remove any bacterial debris. If you have tonsilloliths (white solid drug) learn how to remove it or get help. If this fails, get seen by a physician; you may have a pharyngeal diverticulum or other real problem.