When you think about it there aren't many highways that have steep dropoffs. If you have unlimited time, when not choose different routes even though they might be somewhat out of your way? As another poster suggested, ask for an easy route on the Roads and Routes forum and mention you don't care if it's out of the way. It's worth a try rather than giving up seeing things important to you.
I was also going to suggest that you drive but you said you already do that. Obviously, having control of the wheel doesn't help for you.
Best of luck to you and yes, even seeing a therapist might really help you conquering your fear. I know it helps people who are afraid of flying.