Trivia: The prisoners in the prison scenes of the movie were real-life criminals. The directors decided to use 125 real criminals in the prison they were shooting at to add a level of realism. This is in the DVD special features.
Trivia: At the theater where Evan and the others go, the films Seven and Dumb and Dumber are shown both from the maker of this film, New Line Cinema.
Trivia: The psychiatric hospital where Evan goes is the same on seen in Halloween: Resurrection and Final Destination 2.
Trivia: The diaries used by Evan are the very same ones used by Kevin Spacey in "Seven".
Answer: The point of the movie was that, no matter what he did and how he tried to change things, they always ended up bad. If he went back and did that, something unforseen would have happened to make things terrible. Evan realized that everything bad that happened to them was because of him. He then decided the only safe way to make things right is if he just took himself out of their lives all together. That's the logic the filmmakers went by, if you don't want to accept that, then you will just have to consider it a plot hole.