I have similar problems, my teeth being very yellow and also getting smaller. I like lolite's suggestions and may use some of them myself. I want to add that you may be grinding your teeth in your sleep, which definitely causes them to get smaller(grind them down) and also a dentist told me that it causes loss of enamel and yellowing.
A good tool for night grinding is a night guard. They can be bought for $20 or less or for as much as $500 if you get them fitted to you at a dentist. Also you might get some of Uny's tinctures for nerves, anxiety and insomnia. Even having parasites can cause night grinding, so doing some general cleansing and healing such as taught at this forum would be a good idea.