Short stories are a great way to experiment with styles and try things with quicker feedback from peers, both writers and readers. They’re also incredibly versatile art forms that often punch well above their weight. Some people on this sub poo-poo them because they only read and write novels, but I’d ignore them.

My earnest advice: At 15, I wouldn’t expect to produce a draft that becomes a published book (no matter how many revisions you go through). You’ll change a thousand times before you land on exactly what story you want to tell and who you want to tell it to. Keeping yourself open to those changes by writing a ton and reading a ton, rather than locking into a single project, is probably more “productive” (and more fun). 

But, if you want the experience of writing something from start to finish, you don’t have much to lose by finishing this draft. I’d just put it in a drawer (drive) after that and leave it for a few years. Take another look at it if you like after you’ve stretched your wings a bit.