Any advice with teeth problems?!!!?

So i brush and floss my teeth 2 a day, at night I even use a special tooth paste, but why do I keep getting cavities?!?! My teeth are becoming a somewhat see through and it makes me panic! I don't even drink a lot of pop and when I do I wash out with water after wards and try to drink with a straw. I asked my dentist if this is going to cause me to have terrible teeth since I'm having about 2 cavities every time and if im going to have to have dentures at an early age, he laughed and never answered any of my questions, can anyone help? I'm only 18!