How exasperating. It's like nobody is listening to you. I wish I had an answer. Are you using an HMO? If so, that may be the problem. I had a lump in my throat when I had an HMO and went to doctor after doctor with no one doing a thing about it so I know what you are experiencing. It's ignorant of them to say you are too young to have cancer and to dismiss it like that. You may not have it but it's still wrong of them to assume you don't just based on your age. It certainly sounds like this is what has been running in your family with the heavy bleeding. If you have an HMO, I would keep fighting for your right to a biopsy of the tissue and maybe even threaten a lawsuit if you end up being sick and they did nothing about it. I wish you well. If you don't have an HMO, I would keep calling doctor's offices until someone takes you seriously.
I wish I had more knowledge for you about the condition. If you don't have cancer (which I hope is the case), there is still a real problem with this heavy bleeding. It is not normal and you certainly can't keep going on that way losing blood like this. They MUST do something to cure it.