I should also add that a lot of people think that women cannot ejaculate. Yet many women learn how to ejaculate under the right circumstances. They learn through research, practice, training, and experimenting to see what works. So perhaps the ability to have an orgasm solely from anal stimulation is like that. Perhaps it is just another skill that one might be able to learn and train oneself to do.
I do know that being completely clear can drastically improve the sensations of any anal play, as well as overall relaxation. So in this it is probably necessary to take some plain water enemas at some point on the day that the anal play is to take place, so that you are ready to play when the time comes. This may raise a concern with some people, who will wonder how they are supposed to practice this on a regular basis when enemas on a regular basis is discouraged. Well, the truth is that many people who take enemas on a regular basis are totally healthy, if not healthier, compared to everyone else. Even after decades. Of course you would have a problem with enemas if you have certain preexisting health problems, or if your nutrition is abysmal. Then again, most people's nutrition is abysmal and you should probably look into that, but that is another topic.
I have also read that there are certain steps one must take, over whatever period of time it takes, when learning to orgasm from anal stimulation. It is a long terms process that tends to require commitment. This process applies to both men and women who would like to learn to have orgasms from anal play.
First, always get aroused before you insert something anally. This makes insertions pleasurable. If an object is uncomfortable to insert, you either need more lube or better lube, or more practice with your finger, or a narrower toy.
Second, continue your normal sexual/autoerotic activities but always have a butt plug in at the same time. It is important to have a butt plug in when you get off, to build the link between orgasm and anal stimulation in your brain.
After some weeks, perhaps months, of taking the above steps, edge yourself as close as possible to orgasm through genital play, but cease the genital play before you get off. Instead, try to get off from only the stimulation of anal fucking yourself with a dildo (of course, use a dildo with a base on it so it can't go all the way in and get lost). If you do not get off, then start over with the genital play until you get to the edge again, then stop and try to get off from only the stimulation of anal fucking yourself with a dildo again. Repeat this process every day, or as often as possible, until you succeed. When you do succeed, keep practicing on a regular basis to maintain this new skill.
One more thing, daily kegels are important. However, perform a reverse kegel after each kegel you perform. A reverse kegel is when you unclench, then try to unclench further. The process of kegels and reverse kegels develops the pelvic floor muscles that contract during an orgasm. Kegels without alternating reverse kegels merely builds tension in the pelvic floor muscles, and this tension inhibits orgasm.