So this problem can never be cured. I have had this for a very long time and it is literally killing my social and my working career. I have a congested nose from one side. Sometimes I can see the inflammation of my nasal cavity. However, the blood test showed no sign of sinusitis infection. Flossing and mouthwash are ineffective. Also, I catch cold very easily.
What should I do?

You have went to the ENT and what did they saw was causing the problem.