I think one thing that people don't realise is that there's actually two anal sphincters (even that phrase will make people uncomfortable: one is under conscious control, the other is under unconscious control. The unconscious bit only really cares about safety in this context. Fight, flight, or freeze? Or rest and digest (and crap)?
Shy bowel isn't totally irrational. But the unconscious has got hypervigilant. And why has the unconscious got hypervigilant? Well, it doesn't help that this basic body function is so damned taboo, and the unconscious picks up on taboos over time in order to help you survive in the society you find yourself living in. Which brings us back to square one...
One way into talking about it is to ask the person you want to talk to about your shy bowel if they've got any irrational fears or phobias. And then after they've talked about that, it can provide a way in for you to talk about what you're up against.
Anyway, as my doc would say, "Keep breathing Jonny". You'll get there.