Breakups are almost never easy. Often, there's a multitude of small problems adding up over time that ultimately lead to the end of a relationship. Sometimes, though, it's just one big thing, an action, or a realization, or you know, walking in on your significant other having sex with your roommate. That'd pretty much do it.
Over on Reddit, men talked about the one thing that happened or that they found out that made them end a long-term relationship right then and there.
That being away from her didn't bother me. I went to college two hours away from her, and didn't feel a thing. It wasn't really out of the blue, but it definitely played a big role in my decision.