I too was absolutely paralyzed at the thought of going into a bike shop and looking at bikes. And having the salesperson look at me and think that I didn't belong there because of my weight at the time. I literally felt like an elephant in a bowling alley. I agree with so much has been said so far in this thread : go in as an educated shopper. And that they, the salespersons, are probably even more uncomfortable then you are. Be the first to openly address the issues : will this bike, will these tires do, etc. What are the manufacturer's indications regarding maximum weight, etc. And find a salesperson that will really take you seriously, and that is knowledgeable, really knowledgeable. They are out there. They are passionate about bikes, and if they are really into biking, they are into biking for everyone.

It is all, in the end, about your attitude. I had done lots of research, I had asked around some friends who know more about bikes then I did, and I had done a bunch of phone calls. Some stores I didn't go to just because of how they answered my questions over the phone. Other stores I went too, and left within 5 minutes, because the salespersons obviously didn't know their stuff. But when I finally went into a store, a small neighborhood shop that had a great reputation, it was not nearly as traumatic an experience has I had envisioned. lol The guy had an infinite patience with me, and tweak that seat post a zillion time, and tweaked the handlebar a godzillion time. Actually, the guy trying to help me find the right bike was as tickled as I was to finding me a bike. He took it as a personal challenge I think.
And although I had done some serious shopping, narrowing it down to half a dozen make and models, I was open minded, and ended up buying a very different bike, one I didn't even know about. I test drove it on that small neighborhood street (thank god deserted!), and nothing broke, and the ride felt wonderful and natural. I was a kid again! And I was on my way to a whole new life, literally! I have been to the bike shop a few times since, and I don't feel odd anymore. I own a bike TOO you know, is what my attitude is telling people now. lol
Actually, once you ride your bike, that in itself is a huge confidence booster.
Just go and do it. Don't let anything stop you. Once you will have that bike, you will be so thrilled you did.