Sam, I wonder if you’ve developed a second problem. You might have developed an anal fissure *because* of the excessive sphincter contraction. If that’s the case you’ll probably have to get the fissure healed before you’ll be able to tolerate the biofeedback. If I were in your shoes (and confirmed a fissure) I’d consider Botox injections (in addition to the other measures to comfort a fissure, like sitz baths). They are normally not the first choice for anal fissures, and sometimes it’s hard to get the right amount of Botox (and resulting sphincter relaxation), but Botox *might* temporarily help with both problems (the paradoxical sphincter and the hypothetical fissure). I don’t think the biofeedback would work until the Botox had worn off, though.
All this might be off-target, but if it seems plausible then it might be worth asking Dr. Shen what he thinks. Good luck!