Why do my teeth get stained so quickly after whitening them (on a white diet)? (Photos)

(Whitening before getting veneers on 2 teeth) I've been whitening my teeth for about 1.5 years. I'm bleaching every week, and I'm using Boutique Whitening 6% hydrogen peroxide gel. I'm using my Invisalign trays with this. I wear them for 2 hours in the evening, so I don't eat or drink anything for 8 hours afterwards. I stick to a mostly white diet (cut out wine, chocolate, soy sauce, tomatoes, tomato sauce, tea, coffee, dark berries, curries, Coca Cola, etc). I can whiten my teeth to the shade I want, but they revert back. Any help?