I had a similar issue when I was a kid. It got so bad I would bite my dentist. The solution we came up with (well my parents did) was to take me to a dentist who specializes in children with special needs. This was a good solution because he had a calmer demeanor and temperament and was more patient. He often took the time to explain what was going to happen before he did it, this made me feel better. I used to get anxiety when the dentist would put something in my mouth and then walk away, he learned to never walk away from me.
Not sure if that was of any help to you.