How can I treat long-standing cavities?

Cavities. Damage from cavities can not be reversed. If bad enough, a dentist will drill the cavity out and then place a crown over the tooth. If the cavity is not far along, it cannot be reversed but it can be kept from getting worse. Regular brushing and flossing along with a flouride rinse can help keep the teeth strong and prevent cavities from progressing.
Cavities. It depends on the severity of the decay. This should be examined with xrays. In any case all of the decay needs to be removed in a short amount of time. Months, not over years to just use insurance. The treatment can range from fillings to root canals and even extraction.
See your Dentist. Cavities need to be filled or they progress until you have no teeth left. You may need crowns, bridges or dentures depending upon the severity of the problem. It will not get better with time on it's own.