AdamR: This might be pretty weak advice, but I'm saying it anyways.
Think about the little things that have happened, and that you have done in your life. Ever bought someone a gift? Opened a door for a stranger? Wished someone a happy birthday?
There are countless little things like those I mentioned that people have done for me. Some of them made my day, some have pretty much saved my life. During some of my absolute worst depression, a friend gave me a little paper heart with a bird embossed on it on Valentine's Day. It is difficult to explain why, but the fact that someone actually thought to acknowledge me that day literally may have saved my life. To this day, about 8 years later, she has no idea that that paper heart had any effect on me at all.
My point is... Sometimes when I am feeling like a total miserable failure who can't do anything right, I try to remember some of the little things that I have done for people. I think it is the little things that matter most. When I can remember a hundred of the tiniest gestures that people have done for me - some kind words, a kid giving me a rock he found, a letter someone sent me... Surely a few people have remembered something small that I did for them. Maybe a couple of my tiny gestures have had a major impact on them.
It may be silly to think about that stuff, but it brings me a bit of comfort that I may not be 100% useless.
I'm sorry if that didn't make any sense. I'm tired, and a bit out of it. I'll slowly back out of this thread now...
Honestly, that's not weak, it's a fantastic post. And I can absolutely guarantee that the small things you have done for other people will have made a difference - so don't stop doing them.