The picture is not very clear however from what I can see the darkness that you are noticing is due to caries. It's not too late at all to go to a dentist and have them check to see the extent of the cavity and based on their diagnosis your teeth can be fixed via either dental bonding or crowns.
The best option for you to brighten and improve your smile will be a combination of Porcelain Veneers and/or Crowns depending on how much tooth structure you have lost. Your dentist should be able to diagnose what would be the best solution to give you the smile that you have always wanted.
