Hi. From your description it appears that the nerve supply to your tongue is adequate since it is strong and straight. I do not believe that the slow nasal speech or Rhinolalia aperta is because of the nerves supplying your tongue. The common reason to develop nasal speech in adults is usually due to Velopharyngeal incompetence which means there is an abnormal closure of the velopharyngeal valve (in the throat) due to its mis coordinated speed and precision. It is caused by a neurologic disorders or traumatic brain injury. I do not have details on the incident referred by you, but if it was a head injury then it could be the cause for these symptoms. You need an evaluation by a speech therapist and an ENT evaluation for a video nasopharyngeal endoscopy to evaluate the competency of the velopharyngeal valve. Treatment remains to be speech therapy and exercises.