There are actually numerous reasons why your teeth feels so sensitive to hot and cold substance that you intake.  This is not the symptom that you are experiencing dying of nerves, but this is because of receding gums.  In time, the problem can get worse especially, when you left it untreated.  Brushing too hard can be one of the major cause of gum recession and this can get the root of the tooth exposed.  You can always ask a dental expert or dentist the proper way to brush teeth.  They will be glad that you ask and happy to show you the right way to brush your teeth and give advice on how to treat sensitive teeth.