I have a brother who's older than you, and a sister your age.
You're still young, and you still have a choice.
Don't put yourself down. Just because you're not in today doesn't mean you're not capable in earning a spot, or have less opportunities than we do, yet just because you're older doesn't mean you have more authority.
I'm hard-headed too. Determination got me here, and I think you'll get there through sheer discipline and will to do better.
At one point of time or another, we all face a similar problem or obstacle. The main difference is how we face each ordeal and react to it from future memory, whether the experience be joyous or hurtful.