This is actually far from the first time I've seen Cousins keep his cool under highly frustrating circumstances.
I can't remember when it was, but there was another game I was watching during which Cousins to punked in a huge way (it was a really dirty flagrant foul IIRC) and I was waiting for him to turn around and belt the guy...
You could tell from the look on his face that he was fuming, but he kept he just kinda calmed himself down and just walked away.
There was also a scenario where one of his teammates for hard fouled and got up to try to get in the other guy's face, and Cousins actually stepped in and broke it up and calmed his teammate down.
I've seen stuff like this from Cousins quite a bit in the past year or so. There have been times where I've seen him get visibly frustrated too no doubt, but he seems to really be growing a lot as a person and starting to learn to control himself and not let his emotions get the better of him.
It's a credit to him because he is such an amazing talent and his struggle to control his emotions is probably the one thing holding him back from being an hall of fame level talent. If he can grow as a leader and mature as an individual then there will be no limits to what he can achieve.