How to know if you are fertile or not?

I HAVE HAD THIS PROBLEM FOR TWO YEARS NOW, I CAN HAVE MY PERIOD FOR A WHOLE MONTH AND THEN IT STOPS FOR 2 WEEKS AND THEN ANOTHER MONTH. IM SO SCARED. THE DOCTORS SAY ITS SOMETHING ABOUT MY HORMONES AND THEY GIVE ME BIRTH CONTROLS AND IT STABILIZES, BUT THE THING IS THAT I WAN TO GET PREGNANT AND I CANT BECAUSE IM TAKING BIRTH CONTROLS, AND IF I DONT TAKE THEM I STILL WONT GET PREGNANT, CAN SOMEBODY HELP ME!!!