<< I guess my sadness comes from my being alone. I've always been alone, and it hurts more when others around me find someone, or when I find someone I like whom doesn't like me back or already has someone else. It just reaffirms my loneliness. Which makes me sad. Which makes me wanna just lie in a dark corner and never interact with other people again. >>
I dont wanna sound like I am trying to psycho-analyze you too much or anything, but...
In my experience you need to recognize that the way you are feeling is something of a vicious cycle. You feel unhappy/incomplete/worthless because you dont have anyone to be with. Unfortunately, this means you seek others to validate your worth. But this is self-defeating, because people dont like to hang out with others who are always needing to be validated.
First you need to realize that there is nothing wrong with you! You have value in and of yourself.
If you can first learn to recognize your worth, it makes it so much easier for others to recognize it, and reinforce this feeling.
This especially applies to girls. Once you find happiness individually, you become an asset to a girl, rather than an emotional liability.
Start with trying to learn how to be happy to be yourself, even by yourself. This is possible, although it may take work.
I hope you are able to cheer up!
-Jothaxe