how to get a diagnosis?

i have severe constipation and back and rib and abdominal pain.i have had it for years and years but it is getting worse and worse to the point where i am barely functioning and thinking about killing myself…..i know that sounds crazy but i feel like i am going crazy sometimes.i go to the doctors over and over again but they always say the same thing"we dont know ,just take some laxatives and eat more fiber"…….seriously?
i cannot take it anymore.how can i find a doctor that will really just find out what is wrong with me?i cannot go on living like this………….and i dont have the money to go and get a lot of different doctors to help me so i am so lost and afraid
they have done many tests.
ultrasounds
cat scans
colonoscopy(camera up the rear end…lol)
endoscopy(camera through the mouth to the stomach)
barium enema
barium swallow
and many many more………..
and these tests have yeilded nothing
so what is wrong?
just because they dont know what is wrong doesnt mean that i can continue to go through the pain and agony every day…………………….i NEED help
please someone help me please