About disappointment and self-confidence
Today some of my primary school classmates talked about our monitor, who is an excellent girl all the time, because she got an outstanding grade in the College Entrance Examination in June. But I felt a little sad. I'm always as excellent as her, but this time I'm defeated. (try <em>I'm usually as good as her, but this time she beat me.</em>) Her grade is (or <em>was</em>) 33 scores higher than mine.
So I'm disappointed with myself. But I'm telling myself it's my destiny; though I failed this time, I'm still a unique girl in the world. More importantly, I'll be more diligent and work harder to reach my dream (or <em>dreams</em>). I believe in myself.
I also want to say "Congratulations" to her.
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Your grammar is pretty good, but there were a couple phrases which sounded odd, so I rephrased them. Also, in general, for formal English you can't start a sentence with "so" or "but". Nice job.