One possible version:
Dear Wang Li,
It's good that you know what prevents you achieving satisfying scores in exams.But remember,even
the very best can make mistakes sometimes.So,don't put too much pressure on yourself.
Learning from the errors you've made in the past exams is an effective way to avoid making the same
mistakes next time.If you have problems you cannot solve,turn to your teacher or classmates for advice.
When you attend the exam,plan how to distribute your time according to the number of questions.
Spare a few minutes to review your work in order that you can spot the silly mistakes.
Enjoy the challenge each new exam brings.I believe a good learning habit can eventually help you gain
success.
Edito