I've been using Crest 3D whitening toothpaste for a year and a half and I have braces. Is this bad?

Teeth are not naturally white, they are yellow. The only reason to want white teeth is you are a movie star, and then the studio would send you to a cosmetic dentist that they like. You can not get what a dentist uses to whiten teeth. The chemicals are dangerous. You can only get 3% hydrogen peroxide, which is almost strong enough to damage gum tissues. A dentist uses 40%, which is almost strong enough to explode.

What you do is brush your teeth gently with a soft brush. All you want to do is remove whatever is stuck to the surface because teeth absorb minerals from the saliva. Then floss well to remove accumulated bacteria from the gum line. Dentists are telling people that flossing is more important than brushing, brushing is more important than toothpaste, and whitening is not important at all.