plz help me to correct it and teach me how to make it better

In F.1-2 at LLMC, my excellent results always made me think that I was a gifted child. However, as time goes by, I have accumulated more experence. I have found that my excellent results was a result of luck. Now, I must work even harder for not being outperformed academically. I have borrowed the book not only for improving my English reading skills but for knowing more about gifted-children.

Actually the above answer is pretty good already, but I've got one advice for you. Use active voice as much as possible. Chinese is very different from Eng. We use passive voice to make ourselves sound formal in Chinese. For English, use active voice.
Try to be as direct as possible.

When I was in F.1-2 at LLMC, I thought that I was gifted because I always got good grades. However, as time goes by, I have accumulated more experience and found that my excellence was all luck. Now, I must work even harder for not being outperformed academically. I borrow the book not only to improve my English reading skills but to know more about gifted-children.