What Causes Irregular Periods With Normal Test Results And No Pregnancy?

I m 24 years old and I have been having irregular periods since I was 16. Initially, doctors say that it s normal for teens. Then as I got older, there s no improvement-still irregular. I ve been to several doctors for different opinions, but no luck, they ask me to go on birth control pills. I have done a blood test and my iron, hormones, cervix and uterus are all normal. Pregnancy is negative. I m going towards 7 months without a period and I am getting frustrated. What do you think is the problem or what should I do?