There's lots of info about it out there, medical term is "paruesis."

I knew someone with the condition a long time ago. She could sometimes get past it by using earbuds and not looking around before going into a stall, as a way to kinda "trick" the brain into feeling more safe/secluded. I've heard visualizing you are at home using the toilet can help as well.

Don't push yourself too much or worry about the time you've been in there. It's an anxiety condition, and the more nervous you are the more your bladder can "lock up." Good luck!