Today, I Watched a Movie Called The Flash

Today, I Watched a Movie called The Flash.

It's pretty good.

It's short for It is.

I am not familiar with the Marvel or the DC Universes, so I don't know whether this movie is adapted from thea comic book or if it is original.

The whole set of characters and adventures etc for a set of comic books are called a universe. "a comic book" as you are introducing it. "if it is original" as you are offering an alternative and introducing it.

This movie reminds me a book I pread in the pastviously read.

I forgoet about the name of the book.

Don't need the "about". "forget" present tense as you cannot currently remember it.

The main plot is similar to The Flash,: the protagonist wants to change a bad incident that has just have happened, but every time he went back to the past and tried, it only makesde the future worse.

You do not join two sentence with a comma unless you use a conjunction eg and, or, but. : used as the second sentence gives more specific details about the first.
"has just happened" - has happened is for third person singular perfect tense. "made" past tense like "went back" and "tried to"

At the end, the protagonist is in despair and suicideal.

If you want to commit suicide you are "suicidal".