long abnormal periods

For the last few months my periods have been way weird...well most of my teen years on. I went 3 months without a period and thought I may have been pregnant but than It started and lasted 2 months. It's been doing this awhile. I am trying to get pregnant, and have been trying for over a yr and a half off and on. The doctors keep putting me on birth control to help me but the minute I stop it, it all starts becoming irregular and horrible again. Any ideas what the problem may be? i'm so fed up with every doctor I go to. Anyone else have this problem? I want to find out my ovulation dates but I cant with 2 month periods, than 3 months absent.