Well, backing up a bit. When you say, "fiber"- what fiber were you using?
Irritable bowel syndrome is a diagnosis of exclusion. Basically, when they've eliminated common disorders like ulcerative colitis, Crohn's and other things that can cause problems with the GI system, what is left is a broad diagnosis of "IBS". From there, IBS is broken down into three subtypes based upon whether it manifests as diarrhea (IBS-D), constipation (IBS-C) or mixed (IBS-M).
Because IBS is basically a "we're not sure what causes this" diagnosis, treating it is a challenge. You should get a referral to a GI specialist whose practice is subspecialized to treating patients with IBS. It can take months to figure out the triggers and get a patient with IBS into the right drugs and the right diet that makes their IBS manageable.
There's a new drug on the market- Eluxadoline (brand name: Viberzi)- that is pretty effective for treating intermittent IBS-D. It's really expensive, so the copays can be brutal if you don't have good insurance. The manufacturer offers a copay assistance program that can make it affordable.
There's older drugs like dicyclomine that also help slow the colon down to the point that the patient has better control, although it doesn't completely eliminate the diarrhea.
There's also some research connecting IBS and UC to a neurotransmitter abnormality. Some patients with IBS and UC respond to antidepressants- like SSRIs or Bupropion (Welbutrin).
A GI specialist who focuses on IBS can also refer you to a nutritionist/dietician that can help you with a rotational diet to isolate the foods that make it worse. They may also have you try a FODMAP diet which eliminates foods that produce gas and worsen IBS symptoms.
The most frustrating thing with IBS is that there's no one single cause and it takes trying different things until you find the things that work for you. Unfortunately, in the meantime the day to day symptoms can be miserable. I'm constantly surprised how many patients have IBS and aren't getting the appropriate treatment.