First let me say, I hope you get to realize your dream, and achieve your goals in life; and remember life has a way of working out, at least most of the time.
When I joined the Army, I wanted to fly Helicopters. I couldn't because of some stupid decisions I made when I was a teenager; So I did my time and was discharged. It might seem like a sad story, but I almost certainty would not have met my wife if I had remained in the Army. (Which I would have, if I had been able to fly.) So looking back, which would I choose? Flying of my wife? My wife a 100 times over. We have 4 daughters, which I wouldn't have had. I love my life, and have been greatly blessed. Would I still like to learn how to fly? Sure I would, but I sure wouldn't change the way things turned out.
So go to school, take a variety of classes and see what you like. Enjoy life, (without doing to many dumb things, that get you in trouble) take care of what you can take care of, and don't fret the rest. At least not until you have to.
If it turns out you can't join the military, STAY IN SCHOOL... get a Masters, get a PhD, double major in Philosophy and Physics, minor in Chemistry, or Mathematics.
Then you can always teach college. It's not huge money, but it's not bad, and you can take summers off if you like. Even if you do something else, you can teach part time, or between jobs. What you want is options. I often tell my daughters, if when you grow up you "Want" to be a Wal-Mart greeter, or work at Taco Bell, go for it; but don't get stuck there, because you can't do anything else. Be a truck driver if it makes you happy, but do it with a Masters Degree. (I hope they don't become truck drivers

)
Lastly, though it might seem hard to believe now, for most of us life is a pretty long process (Thankfully) and it sounds like you have your head screwed on straight, so you're way ahead of allot of young people your age already. In light of that, I'm sure things will work in the end.
Take care,
--Tachus