well, one thing that really comforted me the first time that I thought about it (this may or may not apply in your situation) was that the plane that I regularly fly on (American Airlines DFW-Midway) is a POS Fokker F100 (motherfokker! did you know they were still around?! They built the Red Baron's plane!!! :Q) and there's NO WAY any hijacker would know how to hijack that flight. The flights on 9/11 were all 767s or 757s. Of course, it's not like the terrorists are going to try anything with planes again IMO. They've already done that, and it's something that we expect now. No, I think the next time terrorists hit the U.S., it's going to be something different.... I have no idea what "different" is (I wish I did -- I'd avoid it!!) but I think that they won't go for the air any more.
Also, think about it. There are many, many risks in life. Do you stay up all night because you're worried you're going to die in your sleep? Do you stay in your house because you don't want to be shot in a random drive-by? Do you not ride in a car/train/bike/boat because of the chance of an accident? I mean, at one point you have to look at your life and weigh the risks involved in doing your everyday tasks, and just get over it. It's not worth it to suppress your lifestyle because of some meager risk. Live your life to the fullest, and don't worry about the bad things that might happen and over which you have no control. It's all in the hands of God (this coming from a non-religious person).