Just work hard, I know it's a lot of pressure but you will do fine. GCSE's while they are important, are not always everything. I get to sit a 2.5hr English Exam in May (woo!)..
Self Harming is not a good way to vent anger, You have realised this as in this thread, http://www.teenhelp.org/forums/f12-s...-just-started/
We just need to work on it..
You sound like a very athletic person, and should continue doing what you love. Otherwise you may turn into a 'Sheep'. Following everyone else and blending in with the crowd is boring.. If anyone does ask about the scars you don't have to answer.
Don't worry too much about the exams, you will do fine! Believe me..