How do you stay positive and not fall into a deep depression?

I'm C-5/6 24yrs post injury and lately find myself wondering sometimes why am I still here? The few things I that I enjoy are just distractions and aren't enough to keep me happy, and I often think about giving up. It's been really hard watching life pass me by. In my mind I still feel young, like I'm in my 20's. I've never spent 1 night in the hospital since my injury, so I'm as healthy as a quad can be, which I guess is a good thing. I'm just not finding life to be anything more than a burden to other people. I'm so tired of watching and not participating or doing. I hate being constantly stared at and being felt sorry for. I crave the feeling of being seen or thought of as normal. I just don't know where to go from here, or how to feel somewhat happy or normal again.
So, how do you find the strength to carry on when you've been sentenced to the rest of your life in a chair??

How do you stay positive and not fall into a deep depression?