I'm still here...

I have been alive for almost twenty-two years now, and after all that time my heart still beats on cue. My lungs still pump the air when I tell them to, and even when I don't. My brain function has continued without fail. I've never had any major health problems, no hiccups in my mainframe. But how is that possible? How can they still be working? The better question is why? Why am I still here? I don't feel like I am doing anything that makes me worth keeping alive. So why am I? I don't want to be here anymore, but I also don't want to die. Not that it's up to me... I just can't help but wonder, with every step I take, every breath I breathe, and every beat of my heart, when will it end?