I just played my first tournament this past weekend. It was at my home course, a course I've averaged +3 on for the past few months. The pressure of the first tournament, the environment, plus the heat and humidity got to me in the second round. Although I some how shot two strokes better, I finished both rounds at a +9 and +7.
Dealing with the pressure of tournaments is so interesting. My rational mind tells me that it's all the same--still just throwin' some discs around--but it FEELS so different. It's an interesting combination of excitement and formality.
If you've only ever played casual rounds with friends, jumping right into tournament mode can be rough. There are so many aspects of disc golf aside from just throwing discs (courtesy rules, rigorous score keeping, marking your lie, etc). Depending on the friends you play with on a regular basis, those things may or may not be part of your casual rounds.
I find it very helpful to play in weekly leagues at the local course. It's kind of a middle ground: not as much pressure as a big tournament, but slightly more official than a casual round. It's a good opportunity to practice competing in a formal setting. That way when you get to a Big Tournament, it's not such a culture shock that everyone takes it so seriously.