I think you just have to go for it if you don't have an alternate route. I believe that statistically you are less likely to be run over from behind than you are to be sideswiped or doored, but I know that doesn't make you feel any better when a truck is bearing down on you. I will say that I am a pretty timid cyclist, but taking a lane in heavy traffic is less terrifying than I thought it would be, mostly because heavy traffic tends to mean slowdowns, anyway. In terms of driver aggression, I have found that if I take the lane but ride as hard as I can and get back to the right when the bike lane returns, drivers are not horrible to me. They are actually worse if I am lane sharing when there isn't quite enough room -- I think that makes them as nervous as it makes me. If it is clear that I am doing my best to keep traffic moving, maybe drivers are more inclined to be patient while I navigate the area without a bike lane. As I said, I'm pretty slow and timid, but I have only been yelled at a couple of times and neither was in a situation where I was taking the lane.

I always turn on my rear light (on blink) even in the day time if I am going to have to merge with cars. Oh, and signal. That shocks the hell out of them.