Replying to Cheating girlfriend.....

First off I'm 17 and a junior in high school, so I know that I have a lot of life left to live and a lot more chances with women.

 However, this is the shittiest I've possible ever felt, ever (besides family deaths).  My girlfriend of just over 4 months cheated on me.  She left her facebook up on my computer and I saw she was having a convo with her ex.....turns out they fuck all the time and shit like that.  Clicking on that convo was the turning point of the past half year of my life.  Everything went from being perfect to me feeling like I was gonna throw up.  She's been completely normal our entire relationship, and I never would've guessed that she'd do this.  I'm obviously gonna break up with her, and that's about the only resolve I can make as of now.

So my question to Newschoolers is this:  what single, precise moment in your life, whether it be big or small, changed everything?  Whether it be life-altering or just a shitty circumstance like mine.