(This is going to be quite lengthy. I apologize.)
Capitalflush, I suffer from the same problems except that I pretty much never attend social events (maybe twice a year) and when I do, I just linger in a corner or escape to sit outside. Being out in public makes me feel physically ill; I often experience nausea and sometimes even indigestion just from the mere thought of leaving the house (sorry, a little gross). I've only watered my outdoor plants once in the past year because I'm afraid that someone will be looking at me and be revolted at my disgusting body. When it comes to friends and romance I'm the same way as you are. There are times when I can't even bring myself to go to class for the looming prospect of close proximity with other people, which has been quite damaging. I can never answer the phone at work because I'm fearful of sounding stupid or not knowing the answer to a caller's question. Often, I ask myself what my good qualities are and can come up with nothing. My negative thoughts tend to outweigh everything else. I shall most likely never have a romantic relationship of any sort.
When my insecurities start to take hold there are times when I can psych myself into not believing them, but this is rare. Often I just have to try to forget these crippling thoughts entirely and just focus on what is important, like getting an education or keeping good relations with my family. I'm somewhat addicted to sugar and find that when I'm on a sugar high my confidence is heightened and I care much less about what people think of me. When sugar isn't enough I turn to mental techniques. I'll imagine the person that I want to be (a confident, aloof person with nothing to hold them back) and believe myself to be that person. Sometimes I'll "become" a favored character from a book or cartoon. Basically, I'm just playing "pretend", but it does help me to get things done when nothing else works.
I guess the best thing to do might be to think of the things that are most important to you, and of what you desire most. Making a list or diagram of these things might help to get the situation into a simple and focused perspective. Know that your insecurities aren't worth missing out on a fulfilling and enjoyable life. Everyone has their own factors which cause them to question themselves, even just a little bit. It's what prevents us from impulsive actions, which can be dangerous. You are just more perceptive than others. This can be a valuable trait when you don't let it get the best of you. Time to get specific…
-Communication: Your tendency to wonder if what you’re saying or writing is “weird” or if “other people would do this” is quite troubling. It IS good to be unique in your writing and communications for an original piece of work is what will grab a person’s interest or even admiration. Creativity is the only way our society can move forward, we would be nowhere without it. You wouldn’t even have shoes if it weren’t for creativity.
-In public: Everyone is out there to do their own thing. A cashier’s purpose is not to judge or ridicule you. They see hundreds or even thousands of people per day, your few minutes of interaction with them isn’t going to keep them up at night. Besides, you don’t even know those people and you will most likely never see them again, who cares what they think?
-Thoughts: This is nothing to be worried about. One of the greatest things about the mind is that its thinking processes cannot be seen by others. This absolute privacy gives us the freedom to think whatever we like and not worry about how others would perceive it, and you are in no way obliged to tell people everything or anything that you’re thinking. Your concern with seeming “normal” is needless. Everyone’s minds are different; it’s what makes life interesting. Normality cannot exist in a species such as ours. It’s not like we’re ants. We don’t have to be concerned with having uniform thoughts.
-Friends: It’s nice that you care about your friendships so much that you become overly cautious of ruining them. Your friends are lucky that you value them so highly. They didn’t become your friends for nothing; they obviously like you for one reason or another. Friendships are not meant to be hard work, just enjoy them. If you’re not sure whether or not your friends are real just ask them questions that you wouldn’t know the answers to or look at their connections. If they can provide personal connections like parents or other friends then you can ask those people if they know that person (your friend). This should, hopefully, clear up that doubt. If neither of these methods work, then ask me about alternative ways.
-Social events: If other people can go out to meet each other and have fun then you can as well. You're made from the same stuff that they are. And if you really aren’t interested in knowing things about people then why are you worried about this? Social interactivity is a two-way process. I think that you must first decide if you’re really serious about connecting with other people before you start getting twisted into a knot at your lack of progress.
Oh! If you are serious about this, my father recently gave me a book called "Beyond Shyness: How to Conquer Social Anxieties” by Jonathan Berent. He told me that this book changed his life. After reading it and doing the little exercises, he became much more calm and collected during interviews and meetings and was even able to conduct presentations in front of large groups of people with confidence and ease. He said that he wouldn't even be where he is today were it not for that book. I'm excited to have it in my possession; it seems promising. This is just a suggestion, of course. I don't mean to sound like some slimy little salesperson or something. Ick.
Anyway, I don't think that you have schizophrenia at all, just high expectations of yourself. You sound quite intelligent to me, and you seem like a rather delightful person. Take your time and let yourself progress comfortably. I know I have a lot of problems myself, but I want to let you know that you are not alone on this, and it can be overcome without the use of drugs or expensive therapy. I wish you the best of luck. You can do this!
(Heh-heh, it says "Quick Reply" up there but I wrote a freakin' essay.)