You had a normal human reaction....don't beat yourself up over it. Not everyone has the opportunity to stop, evaluate, think, before taking an action and this proves that completely.
Sometimes we all have to make a snap decision and react accordingly. I can remember a couple of times as a kid when we were in the car (back in the mid-70's) and mom would do the arm across the chest thing to us if someone pulled out in front of her. 9 times out of 10 the arm across the chest was worse than us sliding out of the seat! (no she wasn't a bad mom, times were different and the seatbelt thing wasn't like it is now with booster seats and the like!
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Great job for protecting your baby!