Keep practicing and make certain those pedals are adjusted as loose as possible. SPD's aren't my favorite, but a lot of people do like them. Since you've been riding platforms for so long it is just going to take time to re-train your muscle memory about how to stop. Just try to anticipate when you will need to stop and be intentional about your movements.

Penny4, with SPD pedals at least, you automatically unclip when you fall, it just happens. I fell LOTS when I was learning how to use SPDs the first time and both feet always just unclipped on their own. This may not be true for all pedals, I did have this (not) happen once with my Speedplay Frogs, but that may have been an anomaly.