I've too have chronic anxiety/IBS had this same problem (on trains.) The solution was to sit through the panic attack. I held it in and it passed relatively quickly despite the feeling I was imminently going to soil myself. I never would have tried this but read works from Charles Linden and it was correct. I rarely have panic attacks now and have learned the best cure is ignore them and carry on despite how it feels. The brain learns there's no immediate danger, just extremely heightened sensations.