Relief from Botox injections in Masseter muscles last only 3-4 weeks, between 15 and 25 units per side. How can I make it last?

I am a senior citizen and I have been struggling with bruxism/tmj for the last 5 years, ever since I got braces for a short time. An orthodontist told me recently that the mandibular condyle on my right side has deteriorated. Since June, I have been getting Botox in the masseter muscles but after 3 weeks, the relief has all but disappeared. This is fairly expensive. Is there a way to have my medical insurance cover it?