You'll do just fine Laura!
Luckily, the person that makes the final decision is my advisor, and he has already told me that he will let me graduate
Then you just have to worry about not babbling <jk>.
Honestly, what are you afraid off?
You just have to believe in yourself, in what you've accomplished, in what you're defending (say, your thesis).
Don't be afraid of the people standing in front of you. Most of them (public included) are not here to put you down but to help you. The questions they'll ask you are may be harm meant, but usually it's just to point out something so you can either develop, explain or rectify it.
If you must defend you thesis within a defined time though, don't let the questions overflow you, but don't be rude while replying if you think it's derailing your purpose at the time, just say you'll come back to this question later... still try to avoid babbling and go straight to the point, it'll be clearer for the ones listenning, but most importantly, it'll prevent you from loosing your train of thoughts.
Oh! And I think your advisor wouldn't have invited so much people if he wasn't convinced you've already done an A++ job, so, three deep breaths and...
Go get them girl!