It is the involuntary internal anal sphincter over which you have no control. Just like any other muscle in the body, however, it can be worked to influence its control. I used anal dilation, which took time and patience but world for me in the end. Like any tight muscle, it needs a way to relax, and using gradually increasing dilators helped for me and others. You have to do it SLOWLY and GRADUALLY. Doing self massage sounds like a good idea too.
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