Yeah don't beat yourself up, there's no use in doubting every interaction you have. I don't know if you've historically had anger issues or not. "Staying cool" usually isn't a reasonable goal because you WILL get angry, sometimes there's no preventing it. The point is to recognize it happening and figure out what you need to do to manage it without taking it out on others -- its like a self timeout.
I do this when I feel myself getting pulled into an argument with W, I can feel my pulse quicken and see it happening. When you learn to put the brakes on its very empowering.
Evolution has made men scary to women when they are angry -- you can't change that, all you can do is be aware of it. There is no requirement to respond to W in the moment, you can say you need time to think and give her a Timeline for when you'll respond, then make sure to follow up.
Don't read too much into this if you know this isn't a problem for you -- I'm just responding to your post that you were concerned about the exchange
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015