Ha, ha, I had the same issue a few years ago when I was commuting to the train station in street clothes, to go to class. And, it wasn't even an uphill start (which cause me issues even with my Speedplays). My foot just flew off of the pedals when taking off at a crowded intersection, on my old Jamis with campus pedals. There were cars everywhere around me and I got really freaked, in a way I never do with my road bike. However, there have been a few times since then we have rented bikes for city tours while on vacation, and I just have to make myself think and focus while riding with flat pedals and it's fine.
I empathize with you. I had to force myself to cool it while my probable stress fracture healed last fall. Riding did not bother it, while riding, but afterwards, it ached. Since I had to stop hiking and walking and modify my tabata class, I did not want to give up riding. I should have.

