I took a course offered by the League of American Bicyclists that did wonders for my confidence in traffic. I highly recommend this course to anyone who commutes or bikes in any kind of traffic situation. For instance, did you know that you have just as much right as any motor vehicle to be on the road (in most places, except where bicycles are prohibited)? This course teaches you how to bike safely in traffic.
Here's their website:
http://www.bikeleague.org/programs/education/
You can search for instructors in your area. In my area, they put on this course once or twice a year. It cost maybe $40 but it was well worth the money. Although I don't enjoy biking in heavy traffic after taking their bike course, it certainly doesn't rattle me like it used to.