To this day I'm really not sure exactly how and why it went down like it did, but it's the kind of thing where shit was building for a long time and we would just put a bandaid on it instead of confronting the real issues until we just couldn't do it anymore. She was and still is a great person but we wanted different things. She wanted marriage and kids, I really did not, and whenever we talked about those things one or both of us would have a meltdown and so our awesome answer to this problem was to pretend like it didn't exist. Eventually I decided I wanted to do some solo traveling as kind of a soul searching thing and she took it personally and ended it (in an email, after 5 years...)
Also I had some pretty serious emotional problems that for whatever reason I didn't think I could talk to her about, and when I finally told her I was going into therapy she was so happy, because as she told me she had just about given up on the relationship (this was maybe 9 months before we finally broke up) and by the way in a moment of weakness she had kissed a mutual friend of ours. I was too fucked up and exhausted to even care at the time but later on it really bothered me, not so much that it happened but that she had hidden it from me until an extremely vulnerable moment.
So when she later wanted to get back together, I could only think about the stuff like that and not the years of mostly happy times that preceded it, and about how her own problems drained me so much that I felt like I couldn't deal with my own. At the time I was almost relieved, even as I started drinking a hell of a lot and quit therapy.
Now she seems to be a lot better and I'm still the same. (Though I moved away, we still keep in occasional touch over FB and I see her every once in a while.) And although I know that to this day she'd take me back I know that my problems are still there. And while I still love and care about her a lot, I think I killed all the brain cells that were "in love" with her.