If your pain is that high up (a finger inserted all the way in) and I assume doctors have looked in there with some kind of scope and haven't seen anything), it really doesn't sound like a fissure. (Of course, anything is possible). Levator ani syndrome would be one possibility.
Here's an interesting article you might want to check out.
http://www.theiaforum.org/Article_Folde ... l-pain.pdfI don't know what country you're from or what access you have to medical care, but if you can, you might want to consider trying to find a doctor, either a colorectal surgeon who specializes in pelvic floor problems, or a pain specialist who includes pelvic floor pain as part of their practice, to see if you can get anywhere. One way to go about it is to look up physical therapists in your area who specialize in pelvic floor dysfunction and call them up and ask them for a referral to a doctor who treats pelvic floor pain. Pain up that high into your rectum is much much less likely to be due to a fissure than it is some kind of pelvic floor muscle issue.