Part of staying calm is mental, and part of it is physical. The mental stuff will take time, as it requires you to think differently about what you are doing and how you react (playing to enjoy yourself, taking perceived slights in stride, etc.). But there are things you maybe doing physically that are 'accelerating' your tolerance. Holding your breath subconsciously when something is close, sitting with your legs crossed (Yes, this is even possible in a chair or couch. You'll see people cross their ankles.) and clenching are all things people will do that will increase blood pressure. This in turn makes it easier for you emotions to run hot, as you have the chemicals that makes this happen already in your system.
If you find yourself doing some of these things (or don't something else that may cut circulation or cause oxygen deprivation) then you'll want to work on your playing habits. Changing your posture and focusing on breathing techniques while in a stressed environment are two ways you can slowly change how your handle yourself physically that will eventually take root in your subconscious. I'm working on this currently (I've picked up a bad habit of holding my breath sometimes when I play against people in a ranked/staked match, which makes me more agitated and prone to anger or anxiety.) and know it has helped me out in the past.
The mental stuff takes much longer to condition, but you'll find threads all over the place on this forum that have different tips and tricks to try. (Some tips work better for some people than others, so it maybe easier for you to search them out and try what you think is right than have us start throwing them at you.)
