我根据你的意思稍微改动了一些。纯人工翻译，附带中文翻译。不过不知道你的水平究竟是什么程度，这样拿上去你教授会不会识破是别人写的。。。
Any way, i hope that you really walk your talk. otherwise, i will feel guilty for helping you write this letter.
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Dear Mr. Timothy,
Firstly, I am sorry for any inconvenience caused by this letter. I know that you maybe feel hopeless on my performance in your English course. I am also very disappointed by myself. However, I still wish you can give me an opportunity to say something.
首先，我对这封信给你带来的不便表示抱歉。我知道，你可能对我在英语课上的表现已经绝望了，我自己也对自己很失望。但是，我仍然希望你能给我一个机会解释。
As you know, I am a Chinese student. But being different from other Chinese students, I used to be a professional athlete. In China, those teenagers who have sports specialty are sent to dedicated sports school. That is why I have never been to a normal high school before. Therefore, I have not received proper English education before.
你知道的，我是一名中国学生。但是和别的中国学生不同，我曾经是一名专业田径运动员。在中国，那些有体育特长的青少年们都被送到了专门的体育院校。因此，我从来没有进过普通高中念书。所以，我也从来没有接受过正规的英语教育。
Later, I came to Korea for my university. I spent a lot of time studying Korean and my core courses. Therefore, English once again became a serious problem for me. I was very worried about my English. But without a solid foundation, I could hardly make any progress.
后来我到了韩国读大学。大部分的时间，我都用来学习韩语和我的专业课程了，英语再一次成为了我一个严重的问题。我对自己的英语十分焦急。可是由于基础的薄弱，我实在很难有什么进步。
Mr. Timothy, what I want to say here is that, I am not that kind of student who can easily give up. What I need is some more time. I believe that I can catch up through my effort in future. I surely will spend much more time on English. I walk my talk.
Timonthy先生，我现在想说的是，我不是那种轻言放弃的学生。我需要的，只是多一点点时间。我相信，通过我的努力，我一定会把成绩赶上来的。我肯定会花更多的时间在英文上，说到做到。
I really appreciate your understanding.
对于你的理解，我十分感激。
Sincerely,
XXX (你的名字)