How Can Irregular And Missed Periods Be Treated?

i am 27 years and unmarried and i am smoking from 4 years. and i have periods problems(missed periods) from 2 years.i checked the doctor she gave me tablets.it was three months course.then i got periods.but my periods remain regular for 2-3 months.then after again the periods are missed.my hormone levels are normal. then i have PICOS test, which showed that the size of ovary is large but not cysts are found. please tell me what shud i do.is this due to smoking? i have missed my periods from three months now. what do i do? please answer me.