I have decreased my hearing function since I have flu, but after the flu ended the hearing problem still exist, how can I fix it?

Fluid in the ear. You might still have fluid in your middle ear after you had an upper air way infection or flu like symptoms. You need to see an ENT to be evaluated especially when you are still symptomatic. It would takes several months for the fluid to go away by itself and you also have an increase risk of middle ear infection if fluids are there.