I've battled terrible fissures and tried everything from surgery to diet to supplements to laxatives to nitroglycerin cream and more -- you name it. I found that white bread, chocolate, icecream, white meat and a lack of water were food culprits for me.
One thing I've learned the hard way is to eat regularly. With fissures I was afraid to eat, but skipping meals always slowed things down. Eating regularly keeps things moving and, therefore, soft. Unfortunately, you have to be diligent and consistent when dealing with fissures.
And I agree 100% that water, exercise, high fibre and no white flour or sugar is the best way to go. My doctor reminds me that we need about 35grams of fibre a day which is a lot to get without trying. Besides fruit and vegetables, the best sources I've found are the Fibre cereals with 13g of fibre in them and Metamucil.
Also, don't forget to drink water WHILE you eat any kind of fibre.
I found Metamucil initially did not work for me because I was putting too much in my glass of water. Now I use it properly and it is great. Follow instructions when using any supplement.
Finally, I found that my really painful fissures would not heal if I didn't give them some time on laxatives. I know it's bad, but a week or so on Mineral Oil gave me enough of a reprieve from hard bowel movements which allowed my fissures to get better. Then good diet would take it from there.
When you are at your wits end, you do what you have to.