I feel as if this could be referring to many things. Perhaps you miss the very embodiment of youth there, or maybe you miss the innocence (which would be the classic thing, I think) but.. you've left it quite open in terms of what you've missed there. 'you' is awfully vague, anyway.
The ellipses is great, but I wouldn't have minded more depth into what you are missing specifically.
Like you said, the innocence is definitely one thing. But the most important thing that I wish I could go back to is less worrying. I was so carefree back then, I could just go outside and do whatever without worrying what other people thought, and I didn't worry too much about school either. It was really something. :')
Worrying about what I must do motivates me to get it done, however... There's nothing gratifying about it. It's just another thing, another assignment, another project... The worrying I put in alongside the work is never over. Sure, it may have helped me a couple times. But then something else always comes in to ruin what could have been a relinquished worry, and the stressful cycle continues. </rant> Sorry.
I guess I still can't fully appreciate what worrying has done for me. ._.