Aspirations - Writing in my Free Time

So I really want to be a novelist. I love writing, and my friends and family love reading my stories (or so they say). The only problem is, I never can think of enough plot to actually write a substantial book. I get bits and pieces of ideas, but they're just character details, and random stuff that just makes good excerpts. I can't string any of my ideas together. I think I just have permanent writer's block. I'm great at writing short stories, because they don't have much plot. But where's the money in short stories? I mean, sure, I could make a compilation of short stories and sell that, but that won't get me very far unless I sell a few full novels, too. I realize I'm only fifteen, I have the rest of my life, but I can't think of what else I might want to do. I have a head for numbers, but I don't want to do anything that involves research. I like solving Algebra and Physics problems, but I don't know of a job where I can just do that all day. Maybe I'll be an accountant, or work for the IRS and write in my free time... Either that, or I'll just have to marry a man with a job with a big salary, be a stay-at-home mom, and write in my free time... I don't know, I'll figure it out sometime before I die or end up homeless...