I have mvp and mild mr. I have a cavity so I have to go to the dentist soon. Should I be worried for endocarditis?

NO. No, you do not need to be concerned. You do not need antibiotics. Review of studies performed between 1950 and 2006, which included thousands of patients, has shown that there was no benefit of using preventive antibiotics, except in the highest risk patients. You are not in a high risk group if mitral valve prolapse and mitral regurgitation are your only conditions.
Yes. If the dental procedure is likely to produce bacteremia, you should receive prophylactic antibiotic by mouth 30 to 60 minutes before the dental procedures. Your dentist should be able to prescribe an antibiotic for you.