Well, here's one that ended in a good way.
I'll give you the very, very short version.
I knew this girl from the beginning of 4th grade. We were always good friends and dated twice, the second time ending when we went into high school so whe could date everyone she met (which she did). After she broke up with her 500000000th boyfriend that year, we started talking about starting it up again. A bit later, her father had trouble from a misunderstanding regarding his job and moved to a house farther away, and she was forced to change schools.
That summer, we didn't talk a single time. Well, she tried, but I had used the time to think a bit and I realized all the terrible things that had happened because she was around. My entire 7th grade was spent brooding and being quiet and antisocial, which set me on the path to be that quiet and awkward kid for the next three years, she was completely unreliable and manipulative, she started getting in with the wrong crowds which I probably would've been dragged into, and many other things.
But because she moved, I was free and the quality of my life shot up like a rocket attached to a fighter jet attached to five more fighter jets. All in ways at least partly related.
So it almost ruined my life, but because it didn't, I ended up better than ever.