(((((((((((scotlandskye)))))))))))
First of all, I could have written your post myself. I've probably had a lot of those thoughts, word for word. I'm sorry you're suffering so much. Please try to remember that
you didn't do anything to deserve this. Depression is an illness -- you are not to blame for having it.
Also, I know what you mean about not being able to participate in a lot of conversations, and Pomegranate is right, being a good listener is REALLY helpful in building relationships. I didn't realize that I WAS a good listener for a long time -- my depression just convinced me that I was too stupid to get involved in the conversation so I kept my mouth shut a lot of the time, and now most of the people I know will come to talk to me when they have something on their minds because they've determined that I'm a good listener. Weird eh? But it works for me just fine.
What I find works is getting people to elaborate. Like if they tell you where they're from, ask whether they like it, then ask why/why not, etc. My depression makes me too brain-dead to pay much attention to lengthy conversations, but very few people know that I'm depressed, so for the sake of keeping up appearances that I'm more involved in the conversation than I really am, I automatically ask questions or prompt the other person to speak just so they won't see that I can't. The easiest one is "Why?" One word, but it gets you some of the longest elaborations, which in turns saves you from saying much at all.
Also, try not to worry too much about people not understanding your situation. Mental illnesses are very hard for many people to understand and/or accept, even when they try. Just tonight I apologized to my dad because my depression is making me crabby and I've been snapping at my family for no reason the past few days, and I warned him I'd be like that for a while because I can't get in to see the doctor for a few weeks, and all he said was "Everyone gets crabby; it's about self-control. Don't use your depression as an excuse." It bothers me that my own father would think I'm using my depression as an excuse to misbehave, but I've pretty much given up on trying to make him understand it completely. He's tried the past few months, and worrying about making him understand an illness that is as illogical as depression really only makes me feel worse. Just don't go blaming yourself for everyone else's inability to understand, and don't take what they tell you to heart. YOU know you have an illness, and as long as YOU are accepting of and working towards fixing it, that's the important thing. Plus, as has been said before, you always have PC, and everyone here understands what you're going through.