Period for 4 weeks and blood clotting

Hi,
My history is very long. My first period was when I was 10 and since then I have been irregular. When I was 11 I had a period with heavy bleeding for over a month, but when I went to the doctor she said that my period was adjusting. Later when I was 15 some cysts were found on my ovaries and then disappeared, however painful and heavy periods continue, in average it lasted a week and sometimes was twice a month. When I was about 18 I went to the doctors to advise my problem, and the doctor by just looking at me (im overweight) said that I had PCOS and that no further testing was necessary. I didnt even know what it meant until I looked it up but the doctor just gave me birth control pills for it. The birth control pills will somehow control the length of my period but not the pain.Then I went to another doctor and I had an ultrasound and pap smear but didnt find anything and said that I didnt have PCOS. Anyway now I am 24 and with the same problem, but this time I have my period for 4 weeks now, and I have large blood clots, but everytime that I go to the doctors they send me home saying its normal. What could I have and how come no doctor make a proper diagnosis?

[Dshr]

oh thts too bad... one of my frens had similar experiences. irregular, continuing for long long time, excessive bleeding etc. she had birth controlling pills. go to a renownd doctor. cuz u cnt jst adjust on what we say. as ur frens abt a doctor. ur food habit, hormones r major as far as i know. im sorry thts all i cn say...apparently look for a nice doc n improve ur food habit, drink enough..