My heart hurts for all the new mothers with fissures. Having and nursing a baby is hard enough without dealing with the constant pain. It's so cruel that you don't get to enjoy this special time. Please don't think this is a death sentence, or even a life sentence. It's very likely that you can heal completely. If you can, please do take your medications. Take whatever pain killer your doctor will allow. You need to keep the pain to a minimum, as the pain itself can make things worse.
I'm sure you know what to do: you need to keep yourself hydrated, which is even harder for a nursing mother than for most of us. If you take calcium supplements, make sure they don't constipate you. You'll need stool softeners or a very careful diet to make sure you never get constipated or hard stools.
Try sitting on a warm hot water bottle. I know it's hard to just sit, but maybe you can when you are nursing. It's very soothing, and also very healing.
Above all, don't be discouraged if healing takes a long time. It can take weeks or months, and you may find that you have set-backs where you think you are back to square one. Give it time and patience. Believe me, I was cast into depression a few times, when I thought I finally had it beat, and then bam, more pain. But now I'm better. You'll find that the pain decreases and becomes less and less as time goes on. It won't always be like it is now. I haven't had any pain for over 3 months, and even when I do have some pain, it's really more like discomfort, not the misery it was in the beginning.
Good luck!