Abrasion Cavity is what causes tooth sensitivity in most people. It occur at the gum margin and is commonly caused by heavy tooth brushing, among other factors. The exposed underlying dentine or cementum layer is softer than the outer enamel layer, so it mean the cavity quickly deepen if you continue to leave it alone and brush hard.
A simple tooth coloured filling is all it take to relieve your tooth sensitivity. This restoration protect the root surface from being damaged again.
For long term maintenance, you should use a soft toothbrush so that you do not need to replace your filling too often.