I used to write a journal after dinner, and I kept this good habit I kept for over 40 days. I used to write a journal after dinner, and I kept this good habit for over 40 days.

But after the monthly challenge ended, I didn't know what to write about. But after the monthly challenge ended, I didn't know what to write about.

SoNow, sometimes I cheat. Now, sometimes I cheat.

I'm adding "now" to make it clear this is something that developed after the monthly challenge ended. Generally, in written English one should not begin a sentence with "So".

JI just leave a title and some sentences in a journal which is viewable by people who are mutually following each other, and then decide to rewrite it when I have time. I just leave a title and some sentences in a journal which is viewable by people who are mutually following each other, and then rewrite it when I have time.

You have to begin the sentence with "I", otherwise it has no subject and therefore is not a complete sentence. "decide to" is redundant.

This is the main reason I don't follow anyone... Today I found out that more than 4four journals needed to be rewritten. This is the main reason I don't follow anyone. Today I found out that more than four journals need to be rewritten.

"needed" means that in the present, they no longer need rewriting. "need" means that the necessity still exists in the present. Numbers smaller than 10 should be spelled out in writing.

Maybe I should change my writing time to begin after I wake up. Maybe I should change my writing time to begin after I wake up.

This sentence does not make any sense, in relation to the rest of the essay.