Wait. You're only supposed to work on one story at a time?
I have the same problem you do. I'll be working on one story, and then I'll read or think of something that will fit another story, and want to write that one. What I do is write down the idea that I get for the other story in a small notebook. This way, I have the burst of inspiration but I don't feel as if I have to focus on it or else I lose it. This way, I can still write on the story that I'm currently writing but I can still work on ideas for other ones.