I wish I could delete the moments where I ever felt "old" because to think that now is just stupid. I saw the number 24 written on the cover of a book while on the train, and clearly thought these three things -- I am older than that number. I have not read that book. There is so much I have yet to do.
Everyone my age has some dangling worry trailing around after them everywhere that they're somehow not doing everything, that what they're doing is not altogether the right thing, that they are missing out. The doubt is natural, and everyone you know -- yes, even that person -- carries it sometimes too.
Don't give up; not just on what you truly want, but also the seemingly useless and boring things that you have to do "just to get by." Everything has potential, every single opportunity is a piece of your puzzle. It's just a matter of figuring out where it fits -- and revealing the picture it creates.