The Biggest Mistake

Yesterday I took a big mistake.

Although I continued posting English sentences on Lang-8 every day more than 700 days, it ended yesterday.

I wrote an article as always, and I believed that I pushed the "Publish" button.

However, the action didn't work.

I don't understand whether the reason was network failure or my mistake.

Anyway, I'm very sad.

A blank day was created on the calendar of Lang-8.

In fact, however, there is the fact that I have continued learning English every day.

I'm feeling regret for my blunder, but I will keep doing my study from now on also.

最大のミス

私は昨日、とても大きな失態を犯しました。

700日以上、必ず毎日一回Lang-8に文章を投稿してきましたが、それは昨日で終わってしまいました。

私はいつものように記事を書き、「投稿する」ボタンを押したつもりでいました。

しかし、投稿できていませんでした。

ネットワークの不良か私のミスか今となってはわかりません。

とても悲しいです。

カレンダーに空白ができてしまいましたが、実質的には毎日英語の学習を続けているという事実を糧に、これからまた頑張っていこうと思います。

• The Biggest Mistake
• This sentence is perfect! No correction needed!

• Yesterday I took a big mistake.
• Yesterday I made a huge mistake.

• Although I continued posting English sentences on Lang-8 every day more than 700 days, it ended yesterday.
• Although I have been posting English sentences on Lang-8 every day for more than 700 days, the streak ended yesterday.

• I wrote an article as always, and I believed that I pushed the "Publish" button.
• I wrote an article as always, and I believed that I had pushed the "Publish" button.

• However, the action didn't work.
• This sentence is perfect! No correction needed!

• I don't understand whether the reason was network failure or my mistake.
• I don't understand whether the reason was due to network failure or my mistake.

• Anyway, I'm very sad.
• This sentence is perfect! No correction needed!

• A blank day was created on the calendar of Lang-8.
• This sentence is perfect! No correction needed!

• In fact, however, there is the fact that I have continued learning English every day.
• In fact, I have continued learning English every day.

• I'm feeling regret for my blunder, but I will keep doing my study from now on also.
• I'm feeling regretful of my blunder, but I will keep on studying.

• Yesterday I took a big mistake.
• Yesterday I made a big mistake.

• Although I continued posting English sentences on Lang-8 every day more than 700 days, it ended yesterday.
• Although I continued posting English sentences on Lang-8 every day for more than 700 days, I failed to do so yesterday.

• I wrote an article as always, and I believed that I pushed the "Publish" button.
• I wrote an article as always, and I believed/thought that I had pushed the "Publish" button.

• However, the action didn't work.
• However, the action didn't occur.

• I don't understand whether the reason was network failure or my mistake.
• I don't know whether the reason was network failure or my mistake.

• Anyway, I'm very sad.
• This sentence is perfect! No correction needed!

• A blank day was created on the calendar of Lang-8.
• A blank day was created on my Lang-8 calendar.

• In fact, however, there is the fact that I have continued learning English every day.
• The fact remains, however, that I have continued learning/studying English every day.

  "Study" focuses on the process of acquiring new information or skills, and the effort involved in that process.
  "Learn" focuses on the results of that process.

• I'm feeling regret for my blunder, but I will keep doing my study from now on also.
• I'm feeling regret for my blunder, but I will keep doing my study from now on also. [More natural: I regret my blunder, but I will keep on studying (as I have been).]

• Yesterday I took a big mistake.
• Yesterday I took made big mistake.

• Although I continued posting English sentences on Lang-8 every day more than 700 days, it ended yesterday.
• Although I continued posting English sentences on Lang-8 every day, for more than 700 days, it ended yesterday.

• I wrote an article as always, and I believed that I pushed the "Publish" button.
• I wrote an article, as always, and I thoughtbelieved that I pushed the "Publish" button.

  There is nothing wrong with using the word "believe", but "thought" seems more natural. I put a comma infront of "as always" because it is a phrase that is not essential to the meaning of the sentence. You put a comma before to indicate the beginning of the pause and one after to indicate to end of the pause.
  
  An example sentence: This hamburger is delicious. The French fries, on the other hand, are not delicious.
  
  So, the phrase "on the other hand" is not essential to the sentence so we put a comma infront and after to indicate the pause and end of the pause. The reason it is not essentials is because, saying "The French fries are not delicious" is just as acceptable as the latter. Hope this is understadable. If not feel free to ask me any other questions :)

• However, the action didn't work.
• This sentence is perfect! No correction needed!

• I don't understand whether the reason was network failure or my mistake.
• I didon't understand whether the reason was network failure or my mistake.

  Your post is set in past tense so I switched don't to didn't. Unless you still don't know what the reasoning was for the action not working is, it is acceptable to use "don't".

• Anyway, I'm very sad.
• This sentence is perfect! No correction needed!

• A blank day was created on the calendar of Lang-8.
• This sentence is perfect! No correction needed!

• In fact, however, there is the fact that I have continued learning English every day.
• In fact, hHowever, there is the fact that I have continued learning English every day.

• I'm feeling regret for my blunder, but I will keep doing my study from now on also.
• I'm feeling regret for my blunder, but I will keep doing my studying from now on also.

If you have any questions about my corrections feel free to ask :)