Welcome to the IBS forum! I hope we will be able to offer you some help.
When I tried taking fiber I had a real problem with it.... lots of gas, bloating and D. Are you taking too much at once? Or maybe your gut just doesn't like the "cheaper" brands. My doctor told me that Psyllium is much easier on the gut. If you want to continue the fiber therapy then maybe you could try it; it can be found at most health food stores.
I have a co-worker with Diverticulitis and she swears by eating Bran cereal for breakfast everyday. It has something like 5 or 6g of fiber per serving. You may want to take you fiber the easier way, with food instead of a supplement.