From the information you have given i recommend further evaluation of your teeth and tmj by an orthodontist and surgeon. Correction of any misaligned teeth and use of a soft splint would relieve your tmd and associated pain. Also an evaluation by a speech therapist is necessary to identify the core cause for your speech problem.
Next Steps
consult with orthodontist first. (there is no such specialisation called neuro muscular dentist) an orthodontist will be best equipped to evaluate you.
Health Tips
get a second opinion too before you start treatment, since you have already gone through the treatment process unsuccessfully once.