Currently looking for a book with a male teenager protagonist. Possibly a secondary female protagonist.
He meets his future self who has come back to “fix” history. This includes killing people who he thinks are responsible. In the future self’s attempts however, he makes the future worse and forgets that he already attempted to “fix” it. The future self goes insane and the protagonist must find how to stop the damage. He uses an “information helmet” from the future and realizes he can’t become an engineer like he wanted to be.
I distinctly remember the protagonist deciding to “never become an engineer” in order to prevent the technology from being invented.
I read this maybe 2005-2010, it was a school library book (USA). I’m recalling something about a mirror being used to communicate. I didn’t find it on the Wikipedia page for time travel books. Written in English.