Final exam period is the most stressful time for any college student. Yet somehow, despite how I have four final exams coming up in the next week, I don't find myself to be the least bit stressed. That isn't to say that I don't care about college or finals because I've absolutely put effort into studying and completing the projects I've needed to get done, but I don't see the purpose of being stressed out. In the end, you're simply putting extra pressure on yourself, and if you constantly worry that you aren't going to do well on something, odds are you probably won't.
I don't mean to scare you with that statement, but that mindset can be applied to just about anything in life. If you think you're going to lose a match or a fight, you've given your opponent the advantage and you've already lost. If you think your relationship is going to fail, it likely will because that's all that is on your mind. A positive attitude prepares you for a positive experience. Don't just think you're going to do well. Assure yourself you'll do well in anything you do by backing it up: study hard, avoid waiting until the last minute, and come prepared. I know there will be times you'll study like crazy for a test and still do poorly, but hey, at least you weren't stressed out! There wasn't much more you could have done at that point (maybe you just didn't understand the material as well as you thought you did).