Hi fistula generally doesn't cause pain. It should be fissure. You need some good laxative to make stool soft do that it should not hurt the wounded area. Take isabgol husk in Luke warm water after dinner daily. You can try Triphala powder. There are certain ointment to relax the anal sphincter which helps in relieving pain and allows fissure to heal. Do sitz bath three times a day. Feel free to ask any further quary.