There's nothing wrong with short stories; in fact it's often a good thing if it's the only way you can finish anything
But seriously, when I was 15, 16, I was constantly thinking that I could get my great idea written and published by the next year, and like you I was constantly changing ideas and stuff. At the end of the day, you can relax; you have years to get a good idea and write it out and sell it; you don't have to get it done right away. Take your time; learn the skills, foster a lot of ideas and in a few years' time when you're not a hormonal, out-of-control teenager (been there) you'll be able to relax and start looking for the idea you really need. (Just about getting there myself, ish)