However, there are many people who have posted self-introduction onceonce to introduce themselves and never posted again (return?) .

"... and never returned." would also be fine. "self-introduction" is not a normal English phrase as it is in Japanese. I would say it like this.

I have tried to keep on writing mya diary for many times, but I can't do it.

I always described negative feelings, such as regretting tomy break up with my girlfriend, and deemed it meaningless since nobody reads it.

Fortunately, someone wouldill read my diary here and furthermore it is useful for learning English.

However, there are many people who have posted a self-introduction once and never posted again (return?) .

It is difficult for everanyone to make somethingkeep a daily routine (Would "keep on something" be better? ).

I've always described negative feelings, such as regretting to breaking up with my girlfriend, and deemed it meaningless since nobody reads it.

Fortunately, someone wouldill read my diary here, and furthermore it is useful for learning English.

However, there are many people who have posted their self-introduction once and never posted again (return?) .

Both ways you wrote it make sense, it could be either:
"and never posted again"
or
"and never returned"

It is difficult for everyone to make something a daily routine (Would "keep on something" be better? ).

I have tried to keep onup writing in my diary many times, but I can't do it.

I always described negative feelings, such as regretting tohe break up with my girlfriend, and deemed it meaningless since nobody reads it.

However, there are many people who have posted a self-introduction once and never posted again (return?) .

Either works. You could say “posted a self introduction once and never posted again” or “posted a self introduction once and never returned”.

It is difficult for everyonpeople to make something a daily routine (Would "keep on something" be better? ).

“It is difficult for people to make something a daily routine” isn’t the best way to say if.

Using the word “people” is better than “everyone” as everyone of course implies every person which in this context doesn’t read very well as of course there are people who don’t have that struggle.