I believe in the multi-verse approach to backwards time travel. Solves the grandfather paradox. When you go back in time you branch to a new timeline. So if you went back to your high school days, you would be in a world where a version of you exists as a youngster and version of you exists in parallel as the guy who travelled back in time. You’d be two different people and you could talk to your younger self without creating a paradox. When travelling forward in time you stay on the same timeline. Quantum mechanics theorizes that’s what happens.