This all sounds so familiar to me! Well, at least the part where I somehow don't understand how someone gets so attached to me, and I can't truly reciprocate the same feelings. I have also been through a horrible break-up where I simply couldn't comprehend the pain that she was feeling. Though can you elaborate a little further on the concept of a 'fixer'? I haven't really heard that term before..
Basically I'm very nurturing in relationships. As a result, I think that some of my past relationships have been ones where girls with problems/issues gravitated towards me and I them. Like two complementary puzzle pieces. For me, it feels good to take care of someone, but it took me a bit to realize that if there was not a balance, I wasn't going to get what I needed out of the relationship long term and I'd probably end it, which isn't good for either of us. That's exactly what was happening. Luckily that's all been sorted out, -wisdom with experience and all. It's easy to overlook red flags sometimes because companionship feels good and, well, girls can be really cute and stuff, but once you actually notice a pattern it's pretty easy to break it if need be.