I used to have the same problem. What I did is to program myself everytime I was about to visit the dentist, and then I did a permanent programming on myself and now the reflex is all gone.
Also, you can use your toothbrush for example and place it on your tongue for some seconds. Start with one second in an uncomfortable spot on your tongue and with practice you will get used to it. When you get used to it for one second you can try it for two seconds etc. With this, it starts to inactivate the nerves that cause the reflex, but it takes time. Programming yourself gives immediate results.