I tend to divide my stories into two eras: pre-Broken and post-Broken. The pre-Broken stories are all pretty bad, but the one I'm least proud of is one of my oldies called "Silent Desperation." It's about Brian dealing with a painkiller addiction. I wrote it when I was 15 and way too naive to be writing a story about addiction LOL. It wasn't my idea; a friend suggested the drug addiction storyline and somehow talked me into writing it. I had no idea what I was talking about, and my attempts to do research only went as far as looking up drugs on my Encarta '95 CD-Rom, since I didn't have internet access in my bedroom back then. My naivety, combined with my lack of solid research, led to an embarrassingly bad story. If I cared enough to go back and make changes, I would do a lot more research and rewrite the entire thing with more detail and emotional depth. I don't care enough to go to that effort, but at least I learned something about the importance of researching before you write about issues you have no experience dealing with.
At the risk of sounding arrogant, I'm proud of all the stories I've posted since Broken, but I think the part of one of those stories that I'm least proud of is in By My Side when Claire leaves Nick. I knew I wanted them to break up in the middle (so I could get them back together in the end), but I sacrificed her character to do it in a dramatic, Siberia-inspired way ("When I came back, she wasn't there / just a note left on the stairs"). In real life, the strong, compassionate character I'd created would never have done something so cowardly and cruel, but I was obsessed with that song and wanted to work it into the story so badly. I should have just written a songfic to get it out of my system. If I were to go back and change it, I would still have them break up, but I would do it in a more mature, believable way that involved an actual, face-to-face conversation. That story taught me to let the characters drive the plot and not the other way around.