***** NOT A TEACHER *****
I am sure that someone will shortly (soon) check your essay.
I ONLY wanted to make a few respectful points.
I think that university administrators are very busy. Therefore, they may not want to read long letters.
I am no writer, but I think that nowadays people appreciate to-the-point notes.
Let's look at your first sentence: (a) There is no need to write "First of all." (b) I would suggest something (something!) like:
"I wish to thank you for the opportunity to apply for this scholarship in order to study at Oxford University."
Let's look at your second paragraph: There is no need to refer to your country as a "third-world country." The administrator reading your letter already knows that your country is a "developing country." Some (some!) administrators might think that you are trying to make him/her feel sorry for you if you tell him/her that you come from a "third-world" country.
Maybe your second paragraph could be something (something!) like:
"I am Mona Smith, 19. I live with my father (a designer), my mother (a homemaker), and a younger brother, who attends high school. I earned an 0-level equivalency certificate at Mt. Austin High School and am currently studying for an A-level equivalency certificate. I will graduate at the end of this year. I will be the first person in my family to attend a university. [That last sentence means: If I get this scholarship, then I will be the first person in my family to attend a university.]
I respectfully suggest:
1. You should make your writing "tighter." I think that your letter could be 50% shorter -- and more effective.
2. When an administrator sees a very long letter with many unnecessary words, s/he may think that you just sat down, wrote some words that came to your mind, put it in an envelope, and mailed it.
3. Maybe an administrator would be more impressed if your letter showed that it went through several drafts. That is to say, you should keep changing it until you think that you have a letter that has NO unnecessary words.
4. For example, don't tell the administrator that you "performed well." Just list your achievements. S/he will understand that you performed well.
Best of luck to you!
I hope that "Oxford" accepts you.
Student or Learner