Period help? please. serious.?

Its never normal. Like it can go for 2 days to 2 months straight. The doc told me to keep a calendar but I've been kinda forgetful about that. I get clots. Like big red ones. Sometimes black-ish. Its the consistency of jello. they really are big and i feel them coming out. what can I do??!! Whats causing this?! I've had this thing for a year and its never been on a normal schedule.