I haven't been to one, no. I don't understand how standing up or showering could trigger anxiety for me. A shower to me is the most relaxed, stress-free part of my day. Standing up and losing vision leads me to believe it has something to do with blood flow and not anxiety or distress.
You said that larger shower stalls help with your episodes...if the problem is anything other than psychological, how could this be? Perhaps you have some claustrophobia.
If you can exercise at least moderately without the episodes, without any chest pain, etc...then from what you've listed here, in my opinion, sounds psychological. But of course I'm not a doctor. You need a real doctor.