Multiple dimensions of confidence

I am a pretty direct person. If I'm curious about something, I will usually ask; if I'm upset with someone, I will talk to them about it; and my colleagues know that if I have an opinion at work, I'm not shy about sharing it. I'm not always comfortable doing this but I don't think I'm obnoxious about it (or if I realize later that I might have been obnoxious, I usually go back and apologize). But when I read Zandra's story about striking up a conversation with a guy at the gym, my first thought was, "Wow, I could never do that!" I also recently discovered Holly Hoffman's blog and she's had a few posts in the last month about confidence at work and with guys. All of which has me thinking about why it is that in some situations, I seem to have oodles of self-confidence but in other situations (primarily involving people I don't know but would like to), I might as well be back in junior high. Obviously, fear of rejection has something to do with it - it's easy not to take it personally if a waiter gets annoyed with me for saying my steak is over-cooked but detachment is a lot harder if I say hi to someone and they look at me like I've got two heads. At the same time, online dating has taught me to have a pretty thick skin and I think I've gotten pretty good at handling 'rejection' in that forum. So why does the idea of trying to talk to strangers seem so difficult? And I don't mean just guys - anytime I'm in a situation where I don't know anyone (for example, professional conferences), I find it incredibly difficult to meet people. Once I've been introduced to someone, I can make small talk until the cows come home but there's just some switch in my brain that flips from self-confident to insecure when I have to meet new people on my own. Does anyone else have these sorts of isolated insecurities?