I know you are super-frustrated, with good reason! Many of us feel your pain (literally and figuratively), as we've gone through this rollercoaster before as well. It's usually easiest to blame the doctors because we expect them to be the experts - which they are, when it comes to disease - but there are just some things out there that defy explanation. And that's not the doctors' fault.
I went to something like 3 gastroenterologists and a specialty clinic with very little hope and very little cure, until I went to see a primary care nurse practitioner who prescribed three rounds of hardcore antibiotics within six months...and now I consider myself pretty much cured. I have the occasional bouts of insane diarrhea/pain, but this only occurs every 3 months (instead of every 3 hours, like it used to be). Pretty sure it was a parasite for me (based on my travel history to Africa and South America).
Point is...keep trying different things. Experiment with your diet. Don't expect a massive change overnight; dietary changes will only have an effect after several weeks, generally. Perhaps see a nutritionist, or get the book "Eating Right for a Bad Gut." Take your medications EXACTLY as prescribed. Exercise regularly. Definitely get a full colonoscopy when you can. Hang in there. It will get better!