As much as you want to whiten your teeth, all your attempts were brought to a premature end—simply because the process was painful. Some sensitivity while using an at-home whitening kit is to be expected, and misuse of the product can exacerbate a user's sensitivity. But these kits are certainly not intended to inflict pain upon the user. It could be that your dental enamel needs some assistance before your teeth can be successfully whitened.
The Outer Layer
Your enamel is the tough outer layer of your teeth; partially translucent and receptive to whitening. Most of a tooth's structure is formed by a substance called dentin, directly below the enamel. Dentin has innumerable passages throughout it—and these are so tiny that they're microscopic. The passages (called tubules) lead from the outer wall of the dentin to the chamber at its center, which is where the tooth's nerve is found.
Patches of Dentin
When your enamel has worn away, patches of dentin can be exposed. When you apply a whitening gel to your teeth, you're inadvertently applying it to these dentin patches. Trace amounts of the hydrogen peroxide-based whitening agent are pushed through your dentin's tubules to the tooth's nerve, which doesn't exactly appreciate it. You then feel a small amount of pain, which isn't debilitating, but enough to make you discontinue treatment—which is the right thing to do.
With worn dental enamel, you're not a suitable candidate for at-home whitening. You need to schedule an appointment with a cosmetic dentistry clinic. They'll first examine your worn enamel and decide the best form of restoration for it. This may be dental bonding (applying a protective coat of composite dental resin over the worn enamel), or a dental veneer (a tooth-shaped porcelain shell to form the new outward–facing surface of the tooth), or another type of restoration. You'll be asked about how white you want your teeth to be, and the restoration will be manufactured in this color.
Whitening to Match
Once the restoration is in place over your worn enamel, the rest of your teeth will be whitened to match. This allows you to have a smile with the desired shade of whiteness, while also protecting your worn dental enamel from further deterioration. Your smile will actually be a seamless combination of (whitened) natural dental enamel and dental restorations. Don't worry—nobody will be able to tell the difference.
Worn dental enamel can't be whitened using conventional methods, and this is the sort of complication that can only be rectified with help from a cosmetic dentist.
For more info, contact a local dentist.