So you are trying to write a letter in which you ask your teacher if you may look at your exam paper? You have not actually seen the exam paper and how the teacher marked it? If so, here is how I would write the letter:
I was very disappointed to see my score on our English exam. I had hoped that I would do better than that.
I would like very much to understand what I did wrong on this exam so that I can do better in the future. To help me with this, would it be possible for me to see my exam paper and your comments on it? I believe that this would help me learn from whatever mistakes I made.
Thank you very much...
I don't think that you need to talk about whether you deserved the low score yet if you have not seen the paper. I think you should write a letter like this that emphasizes wanting to learn. Your teacher will like that. If you still disagree with the score after you see the comments, then you can go from there.