Continuity mistake: After Evan beats Tommy to death on the college campus, it shows him kneeling over the body, and there is no blood on his collar. The shot changes to Kayleigh screaming, then back to Evan, in the same position, but now with blood splatter on his collar. He had already finished assaulting Tommy, so there shouldn't have been anymore blood spray. (01:09:25)
Continuity mistake: When Evan gets his first nosebleed, during the hypnosis session, the blood on his face changes quite a bit as shots change while the doctor is trying to wake him up. One shot shows two streams of blood, another shows only one, and yet another shows hardly any blood at all. (00:18:00)
Revealing mistake: In the first scene of the film, Evan is writing frantically in a journal and his last words that the camera zooms in on are "Save her". Look closely and you can see that those same words are already indented on the page as indicated by a visible letter "s" and "h" and a period that had already been written on the pad of paper. Obviously they had done some other takes. (00:01:35)
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.