How To Treat Chronic Halitosis?

Hi, I am a 23 year old male and I have been suffering with Halitosis for 3-4 years now. I am struggling to find the cause as it is severely affecting my social life and work environment. I have been cleared by my dentist and brush my teeth at least 3x a day and floss every day. I use mouth wash twice a day and rarely does it even mask the smell. Around 5 years ago I suffered a broken nose and a severely deviated septum. I got septoplasty surgery just last year and it did not help the symptoms, although it did not help my breathing at all either. I still have trouble breathing through my nose and have allergies to cats and dust mites. I was prescribed flonase last month and it has not helped the bad breath either. My doctor recommended omeprazole and I took it for 5 days but I couldnt finish my prescription because of the terrible stomach aches I was experiencing, so I am unsure whether the cause is due to GERD. I have never experienced heartburn or any symptoms that would make me think I have gerd but I do feel like I have a lot of mucus in the back of my throat. Any and all advice is welcome, I am a very healthy guy(no coffee, alcohol, or smoking) and this is severely affecting my life!