When I was 16, I decided to write a long fanfiction as a practice run, just to see if I had the dedication to actually write a novel-length story. It was easier than starting from scratch because I had pre-existing characters to work with, so it was kind of like using a walkthrough when playing a video game --- a tutorial level, if you will 
Anyway, I ended up finishing it after about 2 years. It was just over 300,000 words, and it's actually still up somewhere on Fanfiction.net (although it's nowhere near as good as I wanted it to be, at least not in the beginning). I was really psyched because I actually managed to stick through the whole process....but it was also kind of depressing, because at the end of the day, all I have is this grotesquely lengthy fanfic that I can never really do much with. I mean, I can't publish or sell it. It's just gonna sit up there on that website forever.
Although I did recently start on an original work, and I got a good ways into it, but then I choked myself into a corner on the plot of the second arc, and I had to scrap it (that arc, not the whole story thank goodness) and start over. That left me feeling a little discouraged (it was a long arc...) and I haven't worked on it in about a year. That's also partly due to other developments, though, such as college and musical endeavors.
I just hate not having time to write, really, because I have this awesome idea in my head that I know can work, and it keeps picking up steam the longer I hold onto it --- every now and then a new dimension gets added, or a new twist here, or a new character there, or something --- and I basically have an entire volume completely worked out in my head, I'm just afraid I'll lose it if I wait too long. But on the bright side, if it's managed to stick around for two years and not break up in my head, then I guess that's a good sign
"I'm sorry
For all the things that I never did
For all the places I never was
For all the people I never stopped
But there was nothing I could do..."