Well I don't want to go into too much detail about it since its personal, but as far back as 2-3 months before our relationship ended, she was deeply interested in this other guy. Then when she broke up with me to date him, they essentially jumped right in and started having sex with each other, not even two weeks after me and her broke up. And our relationship wasn't some flimsy thing either. We went through a lot in that year and a half span. It was almost like being married. So that kind of struck a huge blow to me. But what really struck me good was she evaded from telling me why she broke up with me for quite some time. It took me to make up a screen name and pretend to be someone else to finally get answers out of her. All of which were fabrications, that even if they were true she never communicated to me or attempted to fix while we were dating.
Those two things are what brought a lot of damage to me, but as I've said knowing the reasons, whether they're fabrications or not, at least provides me some closure on this whole thing.