Thanks for your query.
Regular use of tobacco and alcohol is very harmful for dental helth.
It can lead to so many serious problems like leuckoplaia,oral submucous fibrosis
So i would suggest you leave these habits at the earliest and visit a dentist
in your area and get the thorough clinical check up done.
For cavity get the tooth restored,if you have sensitivity or pain in the same tooth get an x-ray done and go for root canal
For white patches visit an oral surgeon and get the thorough clinical check up and biopsy
done,to rule out the diagnosis.
Maintain good oral hygiene
Hope this will be helpful.
Dr. Gunjan gupta