How do I stay calm during a game?

Play RAPID games. If you are low on time, play a solid move that does not blunder. Also, focus on improving and sharpening your tactics. When you are low on time, look for ways to activate your pieces and bring position into most suitable style for you.
TACTICAL STYLE: Look for ways to create attack or counterplay.
POSITIONAL STYLE: Activate and maneover your pieces to the point where simple inaccuarcy can lead to tactical attack for you. Bring position into endgame. Study endgames! Endgames are most important.

Also a important note If you have a time increment and if you are low on time try avoid hanging pieces defend all of your pieces there might be a chance you hang something if the piece is undefended just premove.

Take deep breaths to help calm your nerves.