My Neverending Stories...

It's happened again. The teen novel that I started working on again last week has grown and grown in my mind and become - a series.
Since New Year's Day, when I wrote the 10-page scene that fit right in to the novel that I'd started but put aside last year, I've been thinking a lot about the project. I pulled out my notes and story synopsis, and reviewed the manuscript of the beginning chapters. And I remembered that I'd rewrote them when I thought the story wasn't working. The problem was, I'd started writing a vaguely semi-autobiographical story based on my high school years, but after awhile, I noticed that my main character started to resemble the main character from a book I'd tried writing several years earlier, based somewhat on my Freshman year at college. That shouldn't have been a surprise, since the book ideas were autobiographical. So I wondered if the books were supposed to be about the same character. But the fictional background of the character in the college book was very different from the plot of the high school book. And I couldn't figure out how to combine these elements so they would make sense. So I put the project aside.
So now I have the same problem, and I've been trying to figure out what to do about it all week. And I realized that the two stories can be about the same character, at different points in her life - in high school and in college. But then I realized that there should be a story bridging those two points, to show what happened during the time between the high school and college books. And it would have to be three separate books, because at each point in her life she has to deal with and resolve three separate problems, even though there's still a main unresolved problem that would arc over all three stories. And then in a fourth, and last, story, she would finally deal with and resolve that main problem. I know it would be just too long and too complex to write into one book. So now it's become a four-book series.
It's a challenge, but I do want to keep working on it, one book at a time, while keeping the overall story in mind. But now, with four books, it will be a long while before I see the end of the story. And that's my goal this year - to write a book and finish the story. I want to be able to write one book, without having to worry about four.
Why do I keep doing this? I always seem to get story and book ideas that grow and grow out of control. Short stories turn into books, writing exercises turn into scenes from novels, and novels turn into multi-book series.
A few years ago, I unsuccessfully entered a short story in a writing contest. Afterwards, I went over it and realized it just didn't work because there was much more to the story than 3000 words could tell. I realized it was actually a scene that should be in a book. So I wrote a synopsis for the book-length story, and I think it has potential. But by that time, I was annoyed that I'd submitted a mediocre short story, so I put it aside.
I've also started several teen novels, but realized that they are all part of a larger series of books with interlocking characters and settings. That's a very ambitious project, so I put that aside until I have more time and ability to deal with all of them, separately and at once.
This only seems to happen with my fiction writing. When I took journalism classes at NU and NYU, I learned to write tight, put the lead right in the beginning, to cut out anything in the body of the article that doesn't support the lead, and to keep to a strict word count. I wrote many articles following these rules, and got As in all my classes.
But journalistic formulas don't apply to fiction writing. Fiction has its own rules, but a novel can be any length, as long as it's well-written and tells a good story. Many books have sequels, and many are part of a longer series.
I tend to be long-winded. That's just my nature. Anyone who knows me knows I talk a lot, and I can take a long time telling a story.
But somehow I have to learn to rein in my unwieldy stories. I need to give them shape, and a manageable size, so I can see them clearly from beginning to end, and so I can write them from beginning to end.
Or else I'll end up with story after story of Neverending Stories...