You have to taylor make a plan that best suits your body..aiming for soft, consistant stools that do not irritate on the way out either by there size or consistency. I have found a balance for me at the moment that starts with a morning of oatmeal with sultanas, a glass of prune juice with Movicol and one teaspoon of fybergel (smooth) and a cup of coffee. Coffee is a no no I know but It is the best thing for me to get a BM happening. I drink another glass of water as a replacement. The rest of the day I pretty much eat what I like but it is healthy. If I sin and have something like red meat or pizza I usually take another Movicol at night. Mostly I am a good girl cause I know this stuff usually adds another coarse unwanted BM to my day. If I travel I know I will be constipated so I take another Movicol at night, double my water intake and take a laxative at night. We are all a little different. I am prone to constipation rather than diarhoea. I have very little discomfort during BM but the spasms after were horrid. This has been temporarily relieved for me by Nitro and now Botox Injections 10 days ago. Take some time to understand why your fissure(s) were formed and how your body copes with them then take the information you learn from it to develop a program that brings you results.
Fingers and toes crossed. XX Pia

Attack the daybecause already you got great advise and links and already have built your own "fissure attack plan", is one more thing to take into consideration as your fissure is external, make sure that you stop using paper toilet and use baby wipes with not scent or alcohol, very important to be as gentle as you can with the area.
To keep the burning and irritation also lubricate the area before a BM, be careful with the fiber as it can make your stools too bulky, unless you have problems with constipation you want them to be nice and soft.
Well, I wish you the best on your road to recovery and I hope is a fast one.

Also too much fiber may leech out nutrients that your body needs. Once again it may be your diet. Did you just start or stop eating something. Find the culprit that causing those dastardly slits. It can be very tricky to find a cause. Maybe even a food combo that you just stated on is the culprit. I had to brainstorm and take a stock of everything I was putting into my body before i finally figured what it was.
Good luck