I'm so glad you're enjoying it. I don't write fanfic but I write other fic and sometimes it's almost euphoric, isn't it? I mean, when things are falling into place, it's a wonderful feeling.

I think SpaceAnJL's advice is great and you should seriously consider it, but I'm one of those crazy fools who lets the story drag me along. It means I do go down some dead ends, but it's fun to write my way back out (or delete an entire section) and feel my way around until I recognize that I'm on the right track and take off again. And I'm always curious to see where my characters will take me next!

Whatever you do, keep having fun!

Do I ever know that feeling. Word of caution - map the whole thing out, even if it's only a rough outline. Otherwise you can end up just floating in space. You can always pull bits out, change the flow, but having an end to work towards (like JKR's last paragraph) really helps. But ain't it cool to have whole new episodes on the back of your eyelids?