Ugh, I know exactly how you feel. I'm sixteen in five months. Sixteen! Sixteen is supposed to be ages away, not just a few months! And I still feel like the millennium is something that hasn't happened yet. Looking back on when I was younger....you think everything's going to turn out great, don't you? The things that have actually happened to you, never even crossed your mind back then. Then sudden;y you realise just how far along the linme you are. All the dreams that have been shattered, and entirely new ones created. You always look to the future...so much so that you miss the present. You concentrate on the things that are out of reach, not the ones in reach. When I nine or ten...I thought everything was going to be great. I'd go to high school, do brilliantly in exams, get a fabulous career (at that time I wanted to be a paleontologist

), find true love first time round, then live a golden life with friends all around. Of course that was a dream to escape my prsent. The one where I was a shy, unpopular "weirdo". I hated it. But look at what happened instead-
I lost friends, but didn't find new ones
My sexuality came under question
I began not doing so well in school
I became shyer than ever, to the point where I couldn't keep eye contact for more than two seconds
A huge quarrel split my family in half
I got chronic fatigue syndrome
My first serious relationship ended when I was cheated on
And now I sometimes wish to go back to those early years, where everything was far more simple than it is now
But I still have some enthusiasm for life, because there are good things. Very good things. Reasons to live.
Ahem, but this isn't supposed to be about me. Moving on.
I think I've made clear that you are most definitely
not the only person to discover this. I think everyone realises it at some point in their life, and to me, it seems better to realise it sooner rather than later. That big moment where you realise just how fast life flashes by. And I think while you're in that mood, it's good to reflect on things. To make peace with yourself. To change or stay the same. And if this has made a big enough impression on you, then maybe you can start really concentrating on taking in the present. Opening all your senses to it. Experiencing it. Remembering it. It's something many people find hard to do, but they still try. Everyone should try.
I hope some of that babble helped a little.