Don't beat yourself up, in this situation it is a good guess about what exactly is wrong with a fish. Even if you treated early, there is no guarantee that she would have recovered. The fact that you have had this fish for so long, kept it in a good environment, and it got an infection tells me that it was pretty serious, and likely hard to treat at the very best. It could have actually been something internal that no one could do anything about.
Prepare your QT well, and get another YT.