Dear Susan,
I'm glad to receive your letter. I know you are not happy these days .You said you faild the English exam. If I were you I would not worry too much as it was just one exam, it was not the end of the world. There is no point to argue with others about this any more, you lost marke might because you didn't understand the question correctly. To avoid similar thing happening again you could ask your English teacher to discuss the problems in detail. I believe your teacher would be pleased to do it with you.
Cheer up, forget the exam for the moment. As you have told me you would take part in the 800-meter race next week, you need a little practice rather than worrying. I am confident you will win the race as you did last year.
I am looking forward to good news from you.
yours Jack