Train and prepare for shit people doing shit things. Train NOT talking about the past rounds, past mistakes. Train NOT responding and MUTING as soon as you feel dragged towards the emotional mud. Get a stress ball next to you, mute your mic and scream on top of your lungs; anything but flame. Visualize a round as it would probably play out in ranked, NOT as a drill where everything goes right: try to imagine playing 4v5 with no smokes 20 seconds in.
Sleep, hydrate, train. People already provide really good tips on these.
Winning is a privilege, not a right.