Trapped in myself

I'm alone again, but I should be over at Christine's parents’ house. I'm picturing the Thanksgiving meal that I'm missing out on... A big juicy ham, roasted potatoes, steamed veggies and probably some Yorkshire pudding. I wanted to go, I honestly did. But right at the last minute before departure, like a reaper waiting in the corner of my brain, anxiety kicks in. My limbs go numb, I feel weak, nauseous and dizzy. I'm trying to convince myself that there's nothing to be concerned about, there's no reason for me to be worried about anything, but it doesn't work. What am I worried about? Why am I scared?
Christine and I went for a walk around the neighborhood this afternoon, we were passing a new house down the road when a large Saint Bernard took offense to our presence and ran to the end of the driveway, barking angrily. It came right up to us so I crouched down to offer it my hand, letting it know that we were friendly. The dog wasn't having any of this, and continued to bark and growl. Christine got pretty nervous, and circled around behind me as the dog sensed her discomfort and played upon it, following us up the road and directing it's barking towards her. This switched me to defensive mode... I told Christine to keep walking, and stood in front of "Kujo", daring it to try anything. And it just sat there, 6 feet away and growling at me. I called its bluff and told it firmly to "go home" and it did exactly that, turning to trot back to its yard. Why didn't the dog bother me? Why is it that I can stare down a 120lb, angry, drooling dog, but the thought of social interaction makes me want to crawl into a hole? I can drive ridiculously fast on windy roads, hike alone in grizzly country without concern, and climb up on my second story roof on a winter night to shovel snow. But I can't think about going out to a party without breaking into a cold sweat. Sometimes I wish I could switch it around... to be afraid of snakes and heights and other expected things... all to be able to be comfortable while sitting in a restaurant.
It'd be easy if I was missing out on things because I "had the flu", or was "too busy with a project for work". But I'm not, I'm just scared. How do you explain that? "Sorry, I was gonna come over but I got all freaked out and neurotic, for no reason whatsoever, I hope you understand". Yeah, right. I don’t know what’s worse.. Missing out on crap for no reason that I can explain in plain English, or the guilt that follows once I’ve committed myself to sitting home alone.