Sometimes you're so keen for things to work out with someone that you don't see the flaws until it's too late... Was involved with someone like that a couple of years back, on the surface we seemed compatible and things looked promising, but as time went on I began to notice her worrying self-destructive streak.
She'd been through some traumas during her life and I kind of felt that I could help her through it and support her but she damn near dragged me down with her, but I still held out hope until one day when she went completely weird on me. It was probably too late when realised just how much of a sick puppy she actually was and got myself hurt in the process.
Once the pain had subsided though, I never looked back...