Iss it the same dentist this time ?
If it is then they should be aware of the problem and take measures to help .
1. The dentist has seen it before lots of times , dont be embarrassed .2 when you have the stuff placed in your mouth be sitting up , then put your head forward , keep your mouth open as wide as possible . It can help to lift your feet off the chair and try to keep them there to distract you .
It helps to try to breathe through your nose , the material will quickly be set ( although it seems like ages ) .
Some people have a sedative tablet beforehand .
For really severe problems there are other options such as relative Analgesia ( gas and air ) which supresses the gag reflex in most cases altogether . Some have found hypnosis helpful .
Encouragement from the dentist and support from the nurse is also a helpful thing
Good luck
Jenny