Fuzz Nation's book recommendations are spot on. I have both Braden's
Mental Tennis and Gilbert's
Winning Ugly. They are extremely good reads, IMO, and should be a big help with sorting out the baggage in the way station of the mind.
Winning Ugly is especially entertaining for me, because I saw Gilbert play in his heyday. He was often painful to watch, but did a yeoman's job nevertheless. As a commentator, I can quickly get a little too much Gilbert on Gilbert, but his book is good (probably because it predates the emergence of Brad as a media sop). There are things to be gleaned from both texts if you have an open mind. FWIW, I didn't regard it as much a book on strategy as I did a book on mindset.
Gallway's
Inner Game of Tennis is another good read. I'll readily second that notion.
For most of us, I would wager that tennis is an escape from our everyday problems. If tennis itself is a pressure cooker for you at your age, a little bit of guided introspection with a mentor (parent, teacher, coach, clergy, for example) is probably in order to help put things into perspective. I would respectfully suggest that, at a minimum, you do a good double-check to make sure that your expectations are realistic.
Nerves are an aspect of fear, and fear is completely natural. Fear should be embraced, because it indicates that you are alert and watchful when you need to be; that you are prepared to take action. It's a sign that you are very much alive and ready. Breathing and ritual are the enablers that take you from ready to go. These things do work, but you have to know how they fit into the overall puzzle. And you have to have faith, you have to believe in yourself.
You're a young adolescent, and dealing with fear, especially the mounting expectations of adulthood, is part of growing up. Since you are so heavily invested in tennis, both mentally and financially, it is not surprising that the associated anxieties would have an impact on your game.
My fifteen year old son is going through a similar trial right now. He was an accomplished tournament player at fourteen, with a number of appearances in the final and semi-final rounds. Upon being thrust into the Boys 16s, he suddenly found it difficult to just get through the initial rounds, let alone win a tournament. He's finally starting to find himself again, but only after surmounting an appreciable period of self-doubt and deprication. We have spent a lot to time talking through these issues, both in regard to tennis and life in general.
Tennis is just a game. Whether you win or lose is immaterial. What you take away from the experience and learn about yourself is the true value of sport. Where this will take you is for you to decide.
I'm not a psychologist either, just a parent. Parents come in all different flavors, but --- believe it or not --- we've all gone through this in one fashion or another. Just ask.