There are ways to remove the discoloration from your teeth by bonding them and making them look normal. There are several reasons why you have that kind of staining but rest assured you can be helped! As far as the cavity, that tooth may be able to be restored so you should see a dentist asap.
Whitening gel has a expiration of about a year and that is with refrigeration. By now the whitening material has degraded and will be ineffective. I would suggest throwing out the remaining gel and return to your dentist to purchase refills, they are not to expensive.
There are several options for whitening teeth or bleaching them and some methods are more effective than others. Some dentists opt for cosmetic contouring, using a bonding material to cover teeth. If your current family dentist isn't giving you the answers you were hoping for it doesn't hurt to ...