Your problem is remarkably common on this forum, so at least you know that you are not some kind of freak.
First thing I'd recommend is to go to an ENT.
All I can tell you is some of the things I've noted here on this forum, with no guarantees.
1. Some people, including myself, have had a smell that only they can smell. It could smell like dog-poop, old cheese, ammonia, a burning smell, etc. The smell comes on, often, as a result of a cold or sinus infection. Some, not all, of these smells are caused by an anaerobic bacteria thriving in the upper sinuses, and I was able to remove mine with a Flip-Turn Sinus Flush, and other people have had success with it as well
2. Some people had the smell, but were able to get rid of it with long-term antibiotics.
3. Some people had cysts or polyps, which blocked off parts of their sinuses, and created a pocket of infection. One woman, after reading my journal, made an appointment with Dr. Murray Grossan, inventor the Grossan Irrigation Machine, and he removed her cyst and the smell went away. He had seen that problem before.
4. One man did a sinus flush, and some white tofu-like chunks came out. The smell actually got worse for a while, then went back to normal.
5. Many people, particularly the ones who have a strong odor that other people are revolted by, had no change at all after doing a Sinus Flush.
6. One woman found out that the smell was coming from a cracked tooth that her dentist was unable to find, but was discovered by a different dentist.
7. Some smells can come from liver or kidney diseases, or from acid reflux in the stomach.
8. Some smells can come from having a bad mix of bacteria in the intestines, and somehow the smell comes out through the sinuses or lungs. The best solution that I have seen is The Schwarzbein Principle by Dr. Diana Schwarzbein, MD.
9. There is a disease called atrophic rhinitis, which involves the decay of the mucus membranes in the sinuses. I don’t know much about it, that’s what ENTs and Google are for.
10. There are people who have been to multiple ENTs, and the doctors are completely mystified.
11. Finally, there may be a possibly be a fungal infection that doesn’t flush out with saline, but that’s just a hypothesis.