NOT Valium (Diazepam) it's a pretty useless drug as an oral premed. It also has a long clinical action which isn't required.
Now, I've not examined you, this is not an absolute recommendation but more guidelines as to what, in general, would be the correct regime for a patient who needed a little bit of help getting through a dental appointment.
It's my experience that nervous dental patients do much better with Temazepam.
If your weight is more than 50kg, you'd need 30mg about 1hr before your dental appointment. Most people would benefit from a good night's sleep first too, so I would recommend that a patient takes 10mg before bed to help with that. If you're less than 50kg then 20mg is a more appropriate dose.
It's absolutely essential that you bring an escort to the appointment with you, they must do the driving and you must go straight home afterwards and have your escort keep an eye on you for at least the next 6 hrs. Do not go back to work, do not sign any important documents etc etc. You must not drink alcohol for the next 24 hrs.
This drug regimen is perfectly within the scope of a dentist to prescribe and deal with, however if your dentist refuses then your friendly local GP will probably help out.