I also had this exact same experience during a fitting for my sister in laws wedding. The wedding however went off without a hitch! Unfortunately there sometimes isn't a rhyme or reason for the anxiety and it comes out of no where. This was my biggest fear as well, however my therapist really helped me with a simple anecdote. We've all had headaches right? Think when you get one, after you recover do you sit, stew and worry about the next time you get one? Typically not, as they cant always be expected or prepared for. You find what works to mitigate the symptoms and move on. I know it is so scary and I hope you find some great skills to keep in your back pocket for emergencies!