Did you use the same dentist as in previous visits?
Maybe your teeth have been like that for ages.
My back teeth have no enamel as a result of Glandular Fever when I was little.
Otherwise, overvigorous brushing...get yourself an electric toothbrush with a timer on it...about $40 fom Briscoes for a Braun...could protect what enamel you have left.
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