I have the fear of death.it started at the age of 14?

when i was 14 i had this problem and then when i consulted my doctor she told me as i was suffering from severe fever i had that problem.then it was ok.and again after 5 years i am suffering from this problem.i feel as if i am old and im in my last stage of life.im losing interest in many things.those thoughts are just surrounding me all the time.Actually i have a name among my frnds and lecturers that i m courageous and cool.i do.but from month this problem is bothering me.and one more thing is that i m feeling that life is just birth-age-death.please give me suggestions regarding this.