I can't do this anymore..........?

I must post on here atleast once a week. Back in september I went to the doctor for constant urinating and he did a blood and urine test blah blah...nothing. He said my sugar level was fine, my urine had no bacteria or blood in it. He told me it was a prostate infection, gave me a antibiotic, whatever. It helpd a little bit, then it started back up so I went back to him and we did the same thing over again, minus the blood test. NOTHING! My urine had no bacteria or blood, it was completely normal. So he gave me another antibiotic and ditropan and sent me on my way. This antibiotic didn't seem to do crap, but the ditropan helped a little bit. So I went back ONCE AGAIN, and we did the exact same damn thing! No blood or bacteria. HE gave me another antibiotic and they seemed to help. But during this time I was under alot of stress. I had also convinced myself that I had colon cancer because I was having diareha all the time, and if I wasn't having diareha, my stools were very slender,thin mushy looking, so I was freaking myself out to the point thay I tried to take my life. I was rushed to the ER they did an x-ray of my upper and lower back, and my upper and lower front, blood test, urine test, and some kind of heart thing. And once again...NOTHING! This may sound crazy, but would the doctor lie to me about my colon just to get me out of there? Would He actually be able to see my colon and se if there was anything wrong with it? And if so, then what the hell is wrong with me? I've had diareha for what seems like a month now. And i'm still having peeing problems. This is ruing my life I'm constantly stressing about this, and i'm constantly freaking out. Please help me, My girlfriend says it could be irritable bowel syndrome, but I don't know what to think.