Hi. At your age and by your description, I can think of at least two reasons why you may be having tooth sensitivity - faulty toothbrushing or dental cavity. In trying too hard to remove the stains, your aggressive brushing may be harming your teeth. Or neglected oral hygiene has formed a cavity in a tooth. To overcome this problem, one you need to learn the proper technique of brushing and two get the required treatment done.

Tooth sensitivity in a particular tooth can be caused by carious tooth with pulpal involvement. According to your description of problem it seems to be exposed pulp or near pulp exposure.  You need to visit a dentist and checkup for exact cause of problem. Have an X-ray done of infected tooth.  The dentist will decide about treatment like RCT or extraction.  For any further suggestion contact me on docs.animesh@gmail.com. Thanks