Match the opening to the reader
«Dear Sir or Madam, I am applying for the position advertised» works in an application. «Hi! I saw your ad» does not. The content may be identical; the difference is what the reader expects from that kind of text.
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Formal English uses set openings and closings, like «Dear Sir or Madam» and «Yours sincerely». The choice follows from who the reader is and what the text is for.
Ask who will read it and what they expect. The form follows.
Read the feedback for what it says
«The paragraphs jump between topics» is about structure, so you reorganise. «Your sentences all look the same» is about variation, so you change length and openings. Fixing spelling in either case leaves the actual problem untouched.
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Revision means answering what the feedback actually says. Structure comments need reorganising, variation comments need varied sentences, and neither is solved by correcting spelling.
Name the problem in the comment before you start rewriting.
A opens with «Dear Sir or Madam» and ends with «Yours sincerely». A message to a friend does neither. Choosing the level of formality is not about being polite for its own sake, it is about matching the reader and the purpose.
Idioms like «it is raining cats and dogs» cannot be translated word by word, so they have to be learned whole. When you get feedback, read what it actually asks for: a comment about structure is not solved by fixing spelling, and a comment about variation is not solved by making the text longer.