Well to be honest, its your life and probably your call, and you are answerable to your choices.
That being said, knowing that you are the only child, you might have to take into consideration your mother`s advises and suggestions.
Ultimately you have to pursue your goals and dreams and do something you are passionate about.. But you don`t have be in the military to do so.. There are plenty of ways to fight for freedom and stuff. You have to weigh the pros and cons of joining military, seek advises from people who are in military as well as those that left. So you get a rough idea of what to expect if you were to join.. Getting first hand experience would be much more useful than the information you obtain from TV and movies where things could be exaggerated.
If i am you, i would work hard in school. Get good grades, have more choices when you graduate. Maybe by then your mentality would change, and in all honesty, there is only so much you can do in the front line. I am not down playing the contributions of people in military, but i guess the world needs more innovators/ entrepreneurs that do things for the greater good of the world. Consider building a career or business fighting for the freedom of lesser privileged people, animals. Creating ideas or products that changes life.. Probably you will get more sense of achievement and satisfaction doing these.
I don`t know if i make sense, but would be my advice.
Cheers.