Anal fissures are tear in the anal wall. It can't take any more strain. So you have to make sure that you don't constipate. Drink a cup of warm water and do enema gently for few weeks before BM. After enema, lie on bed for 5 mins facing down and then go for BM. That way it will give sometime to anal fissure to heal or not further strain. Eat fruits, drink atleast 6 glasses of water everyday, avoid spices and greasy food. Get enough sleep.