I do not want to write any more long sentence anymores, except this one that I'm writing now.

Another alternative:
“I do not want to write another long sentence, except….”

Today, my university was in theclosed for winter vacation, and I finished my homework.

Universities, like other schools, museums, and stores have times they are open and closed. I suggest using closed for this reason.
.

This will determine that I couldwhether/if I pass this class.

You don't really need a second verb.
"whether" sounds more neutral to me. It just states a fact: "This will determine whether I pass this class."
"if" shows uncertainty in my opinion: It focuses on how important the essay is for you, and makes the reader think about the possibility that you may not pass: "This will determine if I pass this class."

So, I mendcorrected / fixed them and submitted it.

We don't use "mend" for mistakes. We use "correct" or "fix."
"Mend" means to repair, and it has two common uses: we mend clothes, fences, stone walls, etc. and sometimes we say we mend relationships...

I do not want to write a long sentence anymore, except this one that I'm writing now.¶
I do not want to write a long sentence anymore--except this one that I'm writing now.

This is actually a very clever sentence.
I suggest two ways of punctuating it to separate the negative and positive statements.

You could also reword the sentence:
"I do not want to write long sentences anymore, except this one that I'm writing now."
"I do not want to write long sentences anymore--except this one that I'm writing now."

This version reminds the reader that you had to write a whole essay, and it probably had a lot of long sentences in it.
I personally prefer the dash -- between the two clauses because it makes the contrast stronger.

Today, my university wais in the winter vacation, and I finished my homework.

This willcan determine that I couldif I will pass this class.

It had quite a number of mistakes in it.

I do not want to write a long sentences anymore except thise ones that I'm writing now.

My homework is finished.💜

I added a verb to make the sentence complete.💜

Today, my university was in theis on winter vacation, and I have finished my homework.

This sounds more natural in English.

It is an essay written in the English language.

This sounds more natural in English.

This will determine that I couldwhether I can pass this class.

This sounds more natural in English.

I had quite a number ofseveral mistakes in it.

This sounds more natural in English.

So, I mended them and submitted ithem.

This sounds more natural in English.

I dno notlonger want to write a long sentence anymores, except for this one that I'm writing now.

This sounds more natural in English.