Is Tinnitus Retraining Therapy successful?

Hi, I have been suffering Tinnitus (constant high pitched ringing sound in ears) for about 5 years. I have visited 4 different ENT( Ear, Nose and Throat) Specialists and no progress has been made. I have tried and taken several different medicines prescribed by ENT doctors and took some over the counter drugs. There must be good and experienced ENT Specialists somewhere that can give me good advice but I just don't know who else to ask. I would like to try Tinnitus Retraining Therapy which is being offered by some Tinnitus center but insurance will not cover the cost. This 2-3 hour visit itself is very costly ($450 per visit) and they can't guarantee it would reduce this constant buzzing sound. Have you heard anything about this type of therapy?