How to reduce stress during competitive gaming and stay effective?

I thought about how to deal with stress during games. I recently played in a tournament, I tried my best, but I was constantly nervous. As a result, I made several gross mistakes that resulted in defeat. After that, I was so disappointed that I even felt physically tired. I guess I just overextended myself. I would like to understand how others keep their cool and don't let stress ruin the whole game. Maybe someone has some effective methods?