The basic cause of tinnitus is damage to the small hair like structures and nerves in the inner ear which transmit sound. Once damaged, they can't be regenerated. That's why we always hear that there's no cure. Of all the things our bodies can regenerate, why did "nature" forget about these structures?
The secondary cause is our brains which try to make sure all of our frequency range is used. For frequency ranges that have been damaged, the brain creates noise to fill the space. So, we get ringing, buzzing and other sounds that are sound replacements for frequencies we can no longer hear.
Many medical professionals have worked on trying to find ways to rework that brain function, but there are no miracles so far.
The best relief I have found is to override the sound replacement the brain does with other methods of sound replacement. Replace the artificially generated sound the brain creates with other sounds that are more acceptable. Two methods:
First, many people, myself included, find some relief with hearing aids. What hearing aids do is to improve other hearing losses so that the frequency range is fuller with meaningful and enjoyable sound. Often, other naturally occurring sounds can override or divert attention from those the brain is producing. Yet, when in very quiet places the ringing becomes noticeable again.
Second, intentional sound replacement from other sources, or masking. When working alone in my workshop, I listen to audio books or podcasts. Human speech covers a lot of my tinnitus frequencies, effectively masking the constant ringing. Streaming music does the same. Others like to play background music or other types of recorded sound. Part of the effectiveness of these sound replacement techniques is that they give the mind something else to process, the story line of a book or the flow of the music, and thus divert attention from the tinnitus.
Sounds from burbling streams are also an effective "white noise" background, such as the recorded sounds found
here.
No matter what type of sound or music or white noise, sound replacement is the best therapy I've found. Maybe it can help you too.