Bonding or veneers or whitening? Tooth discolouration/porous teeth. (photos)

Hi I have a good set of healthy teeth but have been having trouble with staining of my front four teeth. I used to use alot of harsh whitening toothpastes and i think they have caused damage and porous teeth. I have them cleaned regulatly and they are fine for around 2-3 weeks before becoming stained again. The front tooth looks grey in photos. I have never experienced any trauma to this tooth so i am confused! My dentist did try 'etching' to smooth the teeth, could this have caused this?

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