Being surprised by a high-speed vehicle's presence IMO is the biggest reason those roads "feel" dangerous. If you don't already use a mirror, figure out a way to install one (or two for best measure). When you can get a feel for the road w/ just your peripheral vision it feels much more comfortable to be on the high-speed roads.
If you're a little jumpy to noise (I've become that way due to noise sensitivity), breaking off half a earplug and using that can do wonders. By not covering most of your outer ear you prevent road vibrations from transmitting to your ears (found out this is why musician's ear plugs don't cover the entire ear), and those cars whizzing past you suddenly feel much less intimidating (if you're fine w/ the noise level as it is, you can still try this for a week or two to 'train' your brain that those cars don't scare you). Also brings down road noise from high speed traffic to sane levels that won't damage your hearing. W/ my heightened sensitivity to noise I always bring half plugs w/ me riding, and put them on once I enter high-noise areas.