I actually heard about doing this from my therapist. it was so relieving to tell her my fears, and she even has a private bathroom in her office that she uses because she said she is normally shy, which a lot of people are but never say it. im allowed to use her toilet now!....anyway im really making strides and my confidence is up, especially after taking a "who cares what anyone else things" viewpoint...and realizing that everyone does it...and there are FAR worse things that should be even more of a problem that arent...
for example...vomiting. that should be more challenging since it comes from where you eat, where you kiss, where you talk, and everyone sees your mouth everyday. THANKFULLY, people are understanding because it happens to everyone and you cant stop it from happening when you feel sick
Relate that to poop. its not even half as bad since its done in private for the most part, your behind is hidden by clothing, and nobody will search it out to kiss it.
Another trick.....think of someone you look up to more than anything. or the hottest person you can think of. THEY POOP, THEY HAVE DIARREAH. BE LIKE THEM!!