This is so similar to what I am going through/went through. I had friends in middle school that ditched me when we got to high school, I didn't have any friends for awhile, became best friends with a girl who ditched me when we went to different colleges, I made some friends my first semester in college, they ditched me next semester when we didn't have any classes together, and now I'm back to no friends. So I definitely understand the feeling of worthlessness and feeling like you don't really belong anywhere, and being sick of those kinds of feelings. I'm also having trouble socializing and making friends because I've had so many leave me before.

Anyway, advice. One of the things that kept me from totally descending into the worst place I could fall to was a pen pal. I had a friend that moved halfway through high school and we decided to keep in touch using letters. We are similar in a lot of ways, so we understood each other well. Knowing that there was someone out there that I could talk to, even if it was delayed by a few days due to post, was a great help. I'm also better at expressing myself through writing than talking which is what really draws me to write a letter instead of skyping or something. It seems like you have problems with face to face stuff (I do too) and I think that getting a pen pal would be a great way to socialize without a lot of the social pressures. And it doesn't have to be snail mail (it just feels more satisfying to me), you could email, IM, skype, talk to people on here, etc. It's kind of a long way from randomly meeting people and becoming friends, but it sounds to me like you need to build up some trust with a person or two before you can be fairly social in a wider group setting.