Question about my teeth?

I brush my teeth everyday and go the dentist every 6 months. My problem is my teeth rot from the inside of the tooth to the outer part of the tooth. My dentist says that I may have some kind of disease that may cause this or it may be hereditary. I always have taken good care of my teeth, but lately it cost more to have my teeth taken care of. I will have two or three root cs done and then need to get them done 6 months later because my teeth keep rottening. I thought it was my dentist doing a poor job, so I went to another dentist that told me he would half to do the same procedures. He told me it will cost me a lot for teeth care or I could get them pulled leading to dentures. I am only 29 and I love my teeth, but it cost May more than I have to keep them healthy. Anyone have any suggestions or has this same problem? I don't drink much soda or sugary products.