I've had sensitive and painful teeth, which according to my dentist due to the acidic condtions as a result of a somewhat dry mouth. The enamel just gets eaten away little by little.
As much as I hated the I idea of more flouride, I started using
Fluoridex Sensitivity Relief toothpaste which contains 4x the fluoride of any toothpaste you can buy over the counter. You are supposed to brush it on and leave it on your teeth for 15 to 30 minutes. I usually keep it on about 10 minutes before I wash my mouth out thoroughly.
Well, the stuff works! It works really well.
Unfortunately, I don't think you can buy this stuff anywhere except at a dentist's office. And at around $20 it is expensive for a toothpaste, but it does last a long time.