I've always been more of a one-story-at-a-time kind of writer. That's not to say I only have one unfinished story at a time, but I always have one "main" one and one or more "back-ups" to work on when I'm stuck on the main one LOL. I used to write a lot shorter novels, so I could finish one and move on to the next pretty quickly. Broken's the first story I spent more than a year writing continuously, and now BMS has been going on for over three LOL. Crazy when I think about it, but I'm so attached to it that there's really nothing else I'd rather be writing. I'll be happy to finish it just because it will give me such a huge feeling of accomplishment, but I will miss it too.
I think maybe my thing is that I wrote so many stories the first year I wrote fanfic, I used up a lot of my ideas... and I have a lot of other ideas that have been sitting around waiting for me to finish Broken/BMS, but most of them have plot holes, or I've tried starting them before and haven't gotten very far... there are reasons why I haven't written them before now. So now that you've been writing awhile, maybe the ideas will slow down and you won't be so tempted to jump into new stories all the time. Or maybe you're just so blessed with imagination and inspiration that you'll keep on coming up with unique story ideas and always have a bunch of stories going on at the same time LOL.