I joinattended a short film festival two weeks ago.

You could also say "went to" instead of "attended" for the same meaning.

Short films were being screened one after another from 1pm PM untill 9pmPM.

There was a 30 minute break once inevery two and a half hours.

Before watching them, I underestimated thatem and supposed they would be nothing compared to Hollywoood movies.

ButHowever, after watching them, I became more interested in short films.

They focus attention on one thing, and the story develops at a fast- paced.

In the end, there's a mind twisting endingn unexpected twist.

I watched them for around 6 hours long but never got bored them.

You say "X hours long" when saying the duration of something (ex: the movie is 2 hours long, the flight is 12 hours long, etc.), but not when saying for how long you did something.

When a sameIf another film festival like this is held again, I want to join itattend.

I joinattended a short film festival two weeks ago.

joined insinuates that this is a habitual thing (I joined LangCorrect; you joined to use the site and services) and not just a one off thing (I attended the meeting; went for 1 time with 1 purpose)

Short films were being screened one after another from 1pm tillo 9pm.

to just sounds better here.

until would have sounded okay if you had said "being screened one after another until 9pm"

There was a 30min break once inevery two and a half hours.

So I'm not sure how the festival went, but with the time frame, it makes sense that every 2 and a half hours there was a break.

If there was only 1 break the entire time from 1pm to 9pm I would write "Two and a half hours in there was a 30 min break"

Before watching them, I underestimated them and thought that they would be nothing compared to Hollywood movies.

Underestimated is a verb that really needs an object:

"I underestimated the president"
"I underestimated him"

The second half is your reasoning for underestimated the film and therefore is not an object

But after watching them, I became interested in short films.

I don't think yours was truly wrong, but this sounds better and not so choppy.

They focus attention on one thing and the story develops fast-pacedquickly.

I would say "the story is fast-paced" or the story develops quickly. Fast-paced is an adjective, quickly is an adverb. (:

InAt the end, there's a mind twisting ending.

In vs At
http://learnersdictionary.com/qa/in-the-end-at-the-end

Also, mind twisting ending is correct, other ways you can say it would be:

1) At the end, there's a plot twist.
2) At the end, there's a mind boggling twist.

I watched them for arboundt 6 hours long but never got bored them.

You are stating you have watched them for a certain amount time, don't need long

When a same film festival like this is held again, I want to joinattend it.

"same" and "like this" is redundant

You could also say "When a similar film festival is held again" which sounds even better to me.