Any gastro Dr.'s out there? Is rectal bleeding a sign of something serious?

Well I am 22 years old and have been having rectal bleeding since the age of 15. Over the years it has gradually gotten worse. I have seen 2 doctors and one said an fissure the other hemroids. I have followed there threatment regimens and nothing helps! I am terrified to have a bowel movement because it brings unbareable pain to the point I feel I could pout at times. The bleeding has gotten heavier and heavier, the water in the toilet will be dark red and have no visibility to it (if that makes sense its hard to put into words), when i wipe at times there will be clumps of bright red blood and I will continue to bleed for 10 minutes or so following bowel movements. I guess what I am wanting to know is if I should be concerned, could this be serious, and what it could possibly be other than hemroids or a fissure? If it is hemroids or a fissure then why wont it go away? HELP!!!!