A fissure also may be very slow to heal, especially if it is complicated by thrush infection. It may be necessary to stop putting the baby to the breast and to express the milk manually instead. Usually the milk volume decreases when you do this, but will increase again when the nipples have healed and the baby can suck frequently again. It is reasonable to apply an antiseptic cream as soon as you suspect a fissure, with the hope that it may reduce the chances of bacterial infection making the condition worse.
Another thing that may help is to use a nipple shield (of rubber, or glass and rubber) temporarily. This should reduce the pain and also protect the nipple.