I dated a girl - twice - whose problems were dire. They were entirely of her own construction as well. It was the second time around that I realized that unlike every other girl I'd dated, I couldn't "fix" this one - I told a friend (who I had fooled around with, but not dated) that I felt like I was reaching down onto the rails of a subway to help her up but I could see the train coming and wasn't sure if I could pull her up before she pulled me down.
It was a watershed moment. From that point I started dating defensively. There are people with so much baggage that you can't carry them and the more you try, the more you delay not just your own travels but theirs as well.